Well, if you feel it truly adds to you diversity then put it. Don't be paranoid about including it but do make sure you word it constructively. By that, I mean don't entirely base it off your orientation because, although that may make you unique, relying on that alone is a bad idea IMO and could put the idea in peoples heads that, "Woohoo, he's homosexual, is that all he can bring?"
Having said that, if you REALLY do believe that your sexual orientation has given you insight into the healthcare of LGBT peoples then do include it. You say you haven't done much volunteering in that community so I would have to ask HOW being gay has educated you on the subject. Have you done tons of research, gone to lectures, etc?
So I would say put it in but keep it succinct and make your point quickly as to how it benefits the medical community to have your perspective in it.