First of all, being gay, bi, lesbian, etc. is not a choice. That is simply who you are. For example, for all of you heterosexuals out there, when did you choose to be straight? Contrary to popular belief, homosexuals do not simply wake up and tell the world that they have changed their sexualities.
It's not fair to deny anyone of love. How many times have you heard a parent interrogate their child by asking, "Jimmy, are you sure you have feelings for Sally? She IS a girl, you know." Seldom are heterosexuals chastised for expressing their sexual preferences.
Can you tell when someone is gay by simply glancing at them? Does it really matter in the long run whether your friend is into guys, girls, or both? Will having a transgender coworker make you a sinner? No, no, and no. In essence, just be yourself and let others express their individuality.