I wouldn't be so sure about that. Children, especially the younger they are, are influenceable. I'm not saying it's a given, but, that influence can affect sexual identity outcomes.
They're "significantly more likely to report same-sex attraction, sexual minority identity, and same-sex experience" than the general population.
www.nbcnews.com
CHILDREN OF HOMOSEXUALS MORE APT TO BE HOMOSEXUALS? A REPLY TO MORRISON AND TO CAMERON BASED ON AN EXAMINATION OF MULTIPLE SOURCES OF DATA - Volume 42 Issue 6
www.cambridge.org
Children living with same sex parents have a tendency to identify as homosexual themselves, compared to children of heterosexual parents.
That's not nature, that's influence. Now, no study can be 100% conclusive, one could easily say that the children of gay couples were able to more openly explore their sexual nature than children of hetero couples, I get that, but, estimates range from about 4% to as much as 20% of people identify themselves as LGBT. So, if the children of gay couples tends to have a higher percentage of being gay themselves (I think one study finds between 30% and 57% of daughters of lesbian couples identify as having same attraction), then it could be that the influence of living in a home of same sex parents can indeed affect the sexual orientation of children.
Now, the effect may not be as strong in the classroom, as they are not living with it 24/7, but, if you reinforce it often enough, it can have an effect.