Here's a thought.
Does it matter? Let it go. Let people do what they want, with who they want, as long as no-one gets hurt (if they don't enjoy that). If it doesn't infringe on your relationships, why worry about it?
Edit: Plus, it's not like homosexuality is purely a human thing. It's out there in the animal kingdom. Monkeys do it, dolphins do it, a whole heap of animals will bugger each other senseless regardless of the "normal" procreative urge. So it is part of nature.