I would say a young age, as long as it was research to scare them into avoiding STD's. 
My mom seemed to be under the same philosophy. When I was seven years old, we visited a dear friend of hers whose partner had AIDS. He was in the final stages, I remember, and they were taking care of him at home because it was where he wanted to be. I asked my mom what he had and how he got it, and she told me in simple but detailed terms. A lot of information for a young kid, maybe, but I understood it and it left a strong lesson in my mind. AIDS was never an abstract thing for me again. I think kids are capable of handling a lot more than we give them credit for sometimes.
Do you mean abstract things like a playmate having two mommies -- and that's okay? -- or having two daddies -- and that's also okay?
I'm sick of the hypocrisy and lies we're getting on the pro-Prop. 8 ads here in California. Did you know that "they" are saying that making it legal for gay men and women to marry same-sex partners means that schools will be teaching that it is OKAY for boys to marry boys? For girls to marry girls? RUN FOR THE HILLS. I don't know why schools would be "teaching" that unless they also teach that the norm is for boys to marry girls but that some boys like only boys and some girls like only girls.
I know one thing: Being raised by heterosexuals in a heterosexual environment that vocally denounces homosexuality NEVER DETERRED ANYONE FROM BEING HOMOSEXUAL.
So I don't know how the reverse would or could be true.
Another lie being told is that churches would be liable for lawsuits for hate crimes (!!!) and would start being taxed for not accepting gay marriages. Well, GEE....they aren't taxed if they refuse to marry divorcees. Catholics aren't taxed for excommunicating folks. Mormons and Quakers and Amish aren't taxed for shunning. Why would a church's beliefs about gay marriage be the backbreaker?
STUPID LIES.
