I was just thinking about this over the weekend! Our son (almost 3) does not know the words gay, lesbian, queer, etc. He is mostly focused on having two moms, and that he doesn't have a dad. Although we live in a gay-friendly area, we live in the suburbs with lots of straight families and their kids. I've heard there is another lesbian couple with twins nearby, but haven't met them yet. Anyway, we are really the only non-straight couple our sons sees on a regular basis. We have gay friends with kids, but they live in other states. Our son does remember that these kids have two moms, though, so that is great. When the conversation comes up about no dad/two moms, he can remind us of all his friends that also have two moms. We also like to keep photos around of the kids and their two moms so he has a tangible reminder that there are other kids out there like him.
So far his questions/comments about not having a dad are more about comparison. All of the neighborhood kids have dads, so he likes to tell us that so-and-so has a dad, but he does not. That's when we remind him that he doesn't have a dad, but he has a mommy and mama. We try not to let it stick out in a positive or negative way, just a fact of his life. He also verifies with us that grandpa is mama's dad, and papa is mommy's dad. And then he usually says, but I don't have a daddy? We confirm, and usually return to the statement about him having a mommy and mama. So far he just takes it all in and doesn't seem to really think much more about it. It seems kinda like when we were all kids at a big family event. You know these people are your relatives, but whether it's Aunt or Uncle or Cousin is just too hard to delve into, so you just accept that these people are relatives and leave it at that.
Our son really has good language skills, but the words gay/straight/queer would just be too hard for him to comprehend. He knows he has two moms, but doesn't have any idea what that means in the larger world. And even the kids he plays with (usually up to age 8) have never said anything about us being two moms, so he hasn't had to think about it too much. He definitely has some questions about men in general, and is way more friendly with women than men. He was freaked out by beards for a while (although he never disliked Santa...haha) but we've talked a lot about fear of strangers, and not judging people based on the way they look. That seemed to make him a little more friendly with neighborhood dads and other men in general.
I'm sure we will eventually have discussions about gender and sexuality, but not until he is much older.