how do you know that sephiroth is gay?

Yea he's gay Look at this!!!!

There is absolutely no reason or proof to support Sephiroth being gay. His sexual preference is never mentioned to my knowledge in the games or movie. Although I will note, it's said the creators originally considered making Aerith his lover (Among other ideas). That would hint that if anything, no, he's heterosexual. Fan girls like their Yaoi. That doesn't make it true. :)