I thought the implied romance between my player character and one of the forgettable rivals in XY (not Serena, the other one) was horrible and basically took control of my character and his desires away from me and into the story. And what if I (and presumably I'm projecting myself onto the player character) wasn't attracted to the gender XY was pushing onto me? That whole scene and exchange was jarring and completely unnecessary.
If you have a traditional blank slate protagonist, don't do anything that takes control of the protagonist's presumed emotions, desires, etc. away from your player. You want a flirty NPC? That's fine, they can flirt. But it should be up to the player to interpret what their character is doing or how they are reacting to those advances. The XY scene was so bad because I felt like my player character was returning the advances of whats-her-face without my wanting to. For the first time in a Pokemon game ever, it felt like the story gave the protagonist agency (the ability to think and act on his own, like the other characters), instead of being a puppet or vessel to project myself onto and act through. There's a reason that when control is taken from you and the protagonist's movements are done by the game, they're almost always following/being directed by another character or doing something reflexive that any normal person would do in that situation. It's to keep the protagonist blank so the player can project onto them.
Of course, if the romance involves characters that are not the player, go for it. Extra points if they're not all heteronormative. Heck, even Game Freak is putting in subtle nods to non-hetero or non-gender normative people existing in their universe, like the implied transgender Beauty in the Battle Maison.
