Ooooh this one is tough!! I would be just as okay with it as if they came home and told me they were pregnant/got someone pregnant at a young age. I would obviously be disapointed but I would love them all the same.
HOWEVER, although I could accept my son/daughter being gay, I think it would be slightly more challenging if he turned out like my friend and started cross dressing and being a main "feature" at gay bars.
Although I would still love my child just as much as if he was straight, I would be pretty much heart broken if I found that out(not just the plain gay part, the cross-dressing, wanting to change gender thing).