Well, according to SimsWiki, some sims are more likely to develop a relationship with the same sex then others. I don't know how that works, because at first, sims don't have any preference.
Well, I had one sim that apparently
did have a preference. She had close to a dozen wishes to "kiss X for the first time" in regards to different female sims she was friends with. And it wasn't anything I did, because she hadn't had
any romantic relationships up to this point, gay or straight. Since I am a firm believer in the "you either are or you aren't" philosophy, I determined that she must be. I ended up letting her have her way and her and her partner eventually adopted.
She was charismatic, but wasn't flirty.
