Funny. My son is 8. I actually ran into this issue after he saw two guys kissing on TV a few days ago.
Him: A look of confusion.
Me: "What's wrong?"
Him: "I don't like to see kissing." - This is true. He doesn't even watch a boy and girl kiss.
Me: "You look confused"
Him: "They were both boys."
Me: "And? Do you think that boys shouldn't kiss?"
Him: "I don't want to kiss any boys."
Me: Jokingly I said, "But girls???"
Him: Turning red and doing the embarrassed face, "NO!"
That's when I asked him if he knew what "gay" meant. He said no and I calmly explained that some boys liked to kiss other boys and sometimes they would fall in love. The same with two girls.
Him: "And then they get married?"
Me: "Actually, that's why those two men were kissing. They were just told that they were able to get married."
Him: "Okay." And then he ran off to play Minecraft. And yesterday, we were in the mall and he saw two girls holding hands. When we left, he asked me if they were gay. I said yes, probably, but maybe they were just friends because sometimes girls will do that. Again, he said "Okay" and that was that.
It's a pretty simple thing if you have a brain.
And no, I won't be responding anymore to your comparison to incest. You are equating two very different things to push forward your agenda and it's not going to work.