And Arceus said every Pokémon shall have a gender, and it was good

if you could give a Legendary Pokémon a gender, what gender would you give them, it could be based on looks, or Personality or just whatever gender you wanted to have a Legendary Pokémon of

what legendary, what gender (excluding ones that already have a gender)

Reshiram - female (obviously)
Zekrom - male
Moltres - female
Articuno - female
Zapdos - male
Black/White Kyurem - male
Kyurem - idc
Palkia - female
Dialga - male
Lugia - male

those are my picks...

not sure if this belong in PG or PVP

Oo, I had a thread just like this a while back. I'll answer it all again and add a little explanation for each. Little note: I don't always associate sleekness and grace with feminine Pokemon or strength and burliness with masculine Pokemon.

Articuno: F
Moltres: F
Zapdos: M
MewTwo: M
Mew: F

Articuno and Moltres strike me as very feminine. Moltres has dazzling fire on its head and wings, and its triangular beak and eyes yet rounded body suggest female qualities. It could easily be male, however. I associate Ice with femininity, but also Articuno just looks like something so stern yet so understanding. Zapdos is jagged all over and looks tough-as-nails mean, so I'd call it a boy. The birds can really go either way, though. MewTwo is male for me because... well, it has a male-looking stature. Also, Dan Greene's voice is forever imprinted in my mind for MewTwo. Mew represents an embryo, and all embryos (at least for humans) start as females. It's also pink and looks like an adorable little cat, so there's that.
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Entei: M
Raikou: M
Suicune: F
Celebi: F
Ho-Oh: F
Lugia: M

Entei's ruffled mane and thick stature are very masculine to me. It takes very much after a male lion. I'm also influenced by the movie, in which Entei is some sort of incarnation of the little girl's father. Raikou may be sleek and speedy, but it's facial structure strikes me as male. Raikou is kind of the in-between here, as I could essentially see it going both ways. This is because it seems to be more naturalistic-looking, and while people confuse Entei for either a dog or a lion or Suicune for a cheetah, leopard, or wolf, everyone always associates a tiger with Raikou. Suicune can be nothing other than female because it's the epitome of grace and beauty, and the most beautiful Pokemon ever created in my eyes (even though Entei is my favorite). It has tiny feet, a flowing purple mane, a thin frame, and represents feminine elements such as wind, water, and purity.
And now for Celebi, the little onion fairy. It's another one of those ones that could go either way. But it has a lot of black around its eyes, suggesting eyeliner, and its cute and friendly demeanor definitely seem girlish. Ho-Oh just looks very feminine to me and it's hard to explain why, but I'll try. It seems like it would be so incredibly powerful yet its mysterious, beautiful, and kind of motherly-looking even... that may be because female birds (as well as their male partners) actually care for their offspring. Lugia is big and bulky, that's really the only reason I see it as male. That, and there's a saying that goes, "Mother Earth and Father Sea" or something like that. It could go either way, though.
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Registeel: N/A
Regirock: N/A
Regice: N/A
Jirachi: M
Deoxys: N/A or M
Rayquaza: F
Kyogre: F
Groudon: M

The Regis are perfectly robotic and therefore have no gender to me, no matter what. I see Jirachi as a male for no particular reason at all. Deoxys, I was going to say N/A, but since its eyes look sentient instead of machine-like I'll also classify it as male. It's figure just suggests it, and robotic-looking things tend to strike me as male (unless they don't take after a humanistic figure, like the regis). Rayquaza is female because it's associated with the sky and anything serpentine I usually associate as female. Kyogre represents water, which is typically considered a feminine element, and it is based off of a whale, which, in the case of the orca, is matriarchal. Otherwise whales are looked at as beautiful, gentle creatures that are still incredibly strong. Groudon is hard and brutish-looking, and is associated with earth and fire. It looks angry and unforgiving, thus giving it a more masculine appearance.
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Uxie: M
Mesprit: F
Azelf: M
Darkrai: M
Manaphy: M
Phione: M/F
Shaymin: F
Regigigas: N/A
Dialga: M
Palkia: M
Giratina: F
Arceus: F

I never cared about the Lake Trio and I tend to forget they exist. It was tough deciding Uxie's gender, but I decided it and Azelf are male, and Mesprit female, because of their depictions in Mystery Dungeon. Darkrai looks kind of a like a ballerina with those long legs, but it also reminds me of those whiny emo boys I used to see in school, so yeah. Manaphy looks like a future prince, something that's cute and small now but will grow up to be stern and kingly. There are many, many Phione in the ocean, right? So both genders for them. Shaymin is fluffy-wuffly and wears a flower in its hair, so I definitely see it as girl. In its Sky Forme it's just a tomboyish girl. Regigigas is self-explanatory. Now Dialga is blue and Palkia is pink, but I see them both as male for no particular reason. They look kind of insentient and primordial, like they're just creatures driven by instinct and nothing else, and they're dinosaurs which are inherently masculine. Giratina has "tina" in its name, and that's honestly some of the reason it seems feminine. It converts into a snake-like Origin Form. Yep, snakes are chicks to me! Arceus should probably be both genders, being the God of Pokemon and all, but the flowing wisp of... whatever that is on its head looks like hair, and plus... is it just me or does it look pregnant? The part of its belly with the hoop around it is puffed out and bulging.
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Terrakion: M
Coballion: M
Virizion: F
Keldeo: M
Victini: M
Genesect: N/A
Reshiram: M
Zekrom: M
Kyurem: F

I don't care enough about the Quintuplets to go into great detail about them... Terrakion looks like it has the strength of a bulldozer and Coballion has horns like a male antelope. Virizion... well, just look at it. Keldeo is a masculine My Little Pony doll. Victini was a tough choice, but ultimately it seems like a mischievous little boy who looks a little girly. I can't really explain that one well, sorry. Genesect is, of course, very machine like so it gets no gender.
Reshiram, although many see it as feminine, actually just seems like a very proper, graceful and royal-looking male. Have you ever seen Howl's Moving Castle? This is a weird comparison, but Reshiram is kind of like the Howl of Pokemon... it has tons of feminine qualities but it just screams "MALE!" Zekrom, my favorite legendary besides the Legendary Beasts, looks incredibly muscular and has Zeus-like powers, thus, male. Kyurem represents Ice, and is therefore female.
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Xerneas: M
Yveltal: F
Zygarde: N/A or F

Xerneas has antlers, which, despite this Pokemon being very beautiful and graceful, I can only associate with stags (male deer). Yveltal is female, oddly, even though the Y chromosome only belongs to males. The vulture/bird of prey design, ruff of feathers around the neck, and red coloration solidify my opinion of it. Lastly, Zygarde looks completely primordial but it's also a serpent, therefore it's either female or genderless.

I see all legendaries as male by default, if anything. (Sorry OP, Reshiram is no exception.)

I don't like judging "girly-looking" mons to be female, and the inverse, it plays into the kind of sexism that I find culturally damaging. I like seeing the legendaries as beyond gender for that reason, but I can't help the cultural associations that would pin them as male to me -- if anything -- because masculinity is so often treated as the default state. (After all, this is a franchise like so many others, where the inclusion of female protagonists was presented as an afterthought.)

But yeah. I like them non-gender-constrained. Sex traits are really just for breeding, anyway, and shouldn't be treated like they have genuine inherent qualities beyond physiology. I don't see the point of projecting on 'em like that.

I think this is pretty similar to the other thread, I replied here and realized that my reply was better in the other thread xD So let's move this to PVP and answer what gender we would give to each legendary pokémon, as the OP suggests, and let's discuss genders and breeding for legendaries in the other thread in PGen! :)

-moved

Articuno - M
Zapdos - M
Moltres - F

While I usually see ice as female, Articuno reminds me more of a wise old man than of anything female. Zapdos looks very masculine, and Moltres looks female to me.

Mew - F
Mewtwo - n/a

While Mew is undoubtedly a female in my opinion, Mewtwo is so alien and odd that I don't really see it having any gender.

Raikou - M
Entei - M
Suicune - M

I don't really see any of these being female. Even Suicune looks masculine.

Ho-Oh - M
Lugia - M
Celebi - F

Ho-Oh and Lugia look male to me, but Celebi looks very female.

Regirock - n/a
Regice - n/a
Registeel - n/a

The golems are robots. Which have no gender, being robots.

Kyogre - M
Groudon - M
Rayquaza - M
Deoxys - n/a
Jirachi - F

The Hoenn Trio are like mythological gods and thus male, Deoxys is a robotic alien, and Jirachi looks female to me.

Dialga - n/a
Palkia - n/a
Giratina - n/a

These three are more primal beasts than anything else, and I cannot percieve any of them having a gender.

Shaymin - F
Arceus - n/a
Uxie - M
Mesprit - F
Azelf - F
Cresselia - F
Darkrai - M
Heatran - Both
Manaphy - F
Phione - F

Shaymin looks so obviously female that I cannot say anything but, Arceus is some weird goat robot thing that I cannot assign a gender, Uxie is male for no reason whatsoever while Mesprit and Azelf are female, Cresselia is obviously female, and Darkrai looks manly. With Heatran I'm going to go with the games and say both. I don't care what the show says, but Manaphy is a chick, as is Phione.

Cobalion - M
Terrakion - M
Virizion - M

All 3 of these things are male, seeing as they're based off the 3 musketeers. Virizion's just really flamboyant.

Tornadus - M
Thundurus - M
Landorus - M

They have beards, they're shirtless, and they look like bodybuilders. Male.

Reshiram - M
Zekrom - M
Kyurem - n/a
Keldeo - M
Meloetta - F
Genesect - n/a

Reshiram and Zekrom both look extremely masculine, Kyurem's a robotic turkey and thus neither male nor female, Keldeo's a 4th musketeer and thus male, Meloetta is obviously female, and Genesect is a robot bug.

Xerneas - M
Yveltal - M
Zygarde - n/a

Xerneas and Yveltal both have masculine names and look masculine. Zygarde is a robotic demon snake...thing.

With the large number of legendary Pokemon that we have now, I'm not going to say whether I think each individual one looks more feminine of masculine, as there are still some I feel on the fence about.

If I had to give a select few a gender though, off the top of my head, I'd go with Mewtwo, Entei, Raikou, Jirachi, Darkrai, Victini, and the Swords of Justice quartet as males, and Mew, Suicune, the Lake Guardian trio, Manaphy, Shaymin and Meloetta as females. Most of those are just based off of appearance though, since we don't really get enough interaction with many of them that might give us a better idea of what gender best suits them.

Most legendaries do deserve to have a gender, but some don't. Off the top of my head, one legendary that I could think that shouldn't have a gender is Genesect. Genesect once used to be a real, living, breathing Pokémon. But millions of years ago, something's caused it to go extinct. Team Rocket were able to find its DNA somehow, and with the help of some of their latest technologies, they were able to have it resurrected as a robot. Robots are made out of mechanics, and since machines don't have a gender, Genesect is essentially genderless. Regigigas wouldn't have a gender, either, since it also seems to be a little robotic. Deoxys would be genderless, too, probably—it's an alien, and nobody really knows if aliens have genders or not. d:

Anyway, I'd categorize about twelve to thirteen legendary Pokémon as female—Jirachi, Cresselia, Mesprit, Uxie, Mew, Articuno, Moltres, Latias, Keldeo, Suicune, Shaymin, and Manaphy. Their appearances, characteristics, personalities, behavior, etc. etc. all seem to be quite feminine to me. Every other legendary would be put into the male category, since most of the time, they're portrayed as males.