I don't know what the percentage of gay people is but it's substantial, so you can be quite certain that someone you work with, probably even someone in your own family, is gay.
It's really impolite to speculate about other people's sexual orientation, but if you just keep the idea in your head that there might be gay people around, you might hold off on the careless comments. (And as a hetero, I can assure that it's not only gay people who are offended.)
Even more importantly, many gay people agonize over coming out to their parents, families and close friends. If those parents, family members and friends could just open a little crack in the door to let their loved ones know that they accept them in all orientations, it could ease so much pain. It would also make it less likely that you'll lose someone you care about.
Apologies for the preachy tone.