I don't see why heterosexuals treat homosexuality as if it is some foreign entity, it's just apart of life. Some children even have homosexual parents, yet they aren't psychologically ruined by the idea that their parents exists. Why is it so difficult to explain to your children that some people like the same sex?
I found out about lesbians by watching Friends when I was little (around 5), but I knew that men could be homosexual before that because I remarked something along the line of "I didn't know women could be gay too!!!"
It wasn't awkward to find out about, also my parents never really told me about it just like they didn't tell me about heterosexuality. I worked it out by myself from the TV. Maybe if let more LGBT couples on kids shows which they watch from an early age it would spare you an 'awkward conversion' with your child about human sexuality.