All of Auto's kids (3, 5, 15) know as well as they're able for their ages. The 15yo understands all about poly, as well as queer issues, and is pretty sure that she's monogamous and gay. The 5yo just loves the fact that there are lots of people in his parents' lives that come over a lot and pay attention to him. The 3yo is obviously too young to have any conscious understanding of any of it.
Basically, their approach has always been to just treat it as completely normal. Honestly, the 5yo is more surprised that everybody doesn't have such a complex family. He's far more concerned with whether or not I "liked" his store page on facebook (he's started making and selling plastic beaded jewellery).
It's like the "The Talk" idea. I don't think it should be The Talk. I think there should be frequent and informal age-appropriate discussions about sexuality, orientation, gender expression, relationship forms, etc., throughout childhood and adolescence. If you make it The Talk, it's like this big taboo thing that's inappropriate to discuss, and that makes it seem dirty and wrong. If you make it natural and part of everyday life, then it's not that big of a deal, and they won't need to go out and try it just to rebel.
I agree with GG that he was probably being literal when he asked if you loved Nudge more than Daddy. He would probably have been satisfied with "Nope, I love them the same amount, just like I love you and your sisters the same amount, but a different way than I love Daddy and Nudge."
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker