If you're going to relate Pokemon to real animals (by insisting that they reproduce sexually), you may as well include the rest of the science that goes along with reproduction.
Note: Pokémon only very loosely obeys
any laws of science. If we're going to go with "inter-special breeding is impossible the way they put it," we might as well also address the fact that in this world, a foot-tall rodent can unleash several hundred volts of electricity, that instantaneous (painless) transformation from a five-foot-long serpent (Dratini) to a thirteen-foot-long one (Dragonair) is completely possible, that matter can be converted into energy and
then into data that can be stored on an ordinary computer, and that time/space travel is also possible not only by the magic of the pokémon themselves but also that world's technology (i.e., Time Capsule).
Point is, I think Pokémon fully acknowledges that for the sake of the enjoyability of the game, it suspends a
lot of laws of science and realism. That and it's for the sake of simplicity. The reason why a hatchling is the same species as the mother is because game cartridges probably can't handle the hundreds upon hundreds of different possible combinations. The reason why breeding groups are so wide, meanwhile, is so that breeding is made easier for the players. (That is, the player isn't restricted to having to catch more than one member of a species. So long as the player has one member of the same
egg group, they don't have to tear their hair out over trying to get a child with the moves and nature they want.)
That being said, the Celebi thing's really just another way of looking at it. It hasn't actually been explicitly proven in canon (yes, 'dex entries considered). Neither is actual sexual reproduction. As I've said in another thread, the game designers deliberately make it ambiguous. In terms of logic, the breeders probably don't tell the hero avatar about the birds and the bees because the kid's ten years old. Sure, we probably knew about it at that age, but try imagining being told how things work by people old enough to be your grandparents. "We didn't see it happen" is probably simply an excuse. In terms of design, it's mostly so we can come up with our own conclusions.
(As a side note, while it's unlikely that a breeder would actually not know how breeding goes down, I think it'd be hard to explain not noticing Celebi hanging around one's pokémon, especially given that -- in the movies, at least -- it emits a song right before or right after it makes a jump through time. So, one could think they're deliberately not telling the hero something, but this concept could apply to really any argument one could offer as to why no one seems to know how pokémon breed.
However, it should also be noted that Elm didn't know that the mystery egg was actually a pokémon egg at the start of the GSC games. Remember, his biggest discovery was that pokémon lay eggs in the first place. This is a well-established, regional researcher, no less.)
In other words, there's really no right or wrong answer. Celebi's one way of looking at it, but please don't say anything else is impossible. Especially if you want to call it a violation of the laws of science (considering how much else in canon is).
Sorry if this sounds like derailing the topic too much. I'm also in part justifying the WTFery when it comes to breeding groups here. As in, it partly makes things easier for the players, and it's probably also because the games just don't make scientific sense. Hence hilarity like Wailord x anything, really.
