Same reason people always fall in love -- they're attracted to the other person for whatever reason. There's no special reason for it, nor does the fact that Seiya is really a girl mean it shouldn't be possible. Remember, Michiru and Haruka (Sailorneptune and Sailoruranus) are lovers. It's not that big a deal in anime.
Originally posted by Takiko_4ever
i personally think that Seiya fell in love with Usagi becasue
he...er...or she thought Usagi Was Kaguyuu aka Sailor Fireball...
i don't know? it's just an educated guess.
It's spelled Kakyuu. I think you're mixing the name with Kaguya, the Snow Princess from the S movie.