Weirdly enough, in this game homosexuality is something you'll learn. Homosexuality is considered simply as a matter of taste. Your partner can become a lesbian through training, i.e. repeated sex acts with women. Just like you can learn to like the taste of beer to the point where you prefer it over other beverages.
Not sure if it was your intention, but you raise an interesting point. It would make a lot more sense if Homosexuality was a permanent trait like Quick or Clever. It would make gameplay more restrictive though, but it's not always a bad thing.