Kids will ultimately not find non-heterosexual relationships confusing at all (just ask same-sex parents). If a kid has never met a gay couple before, they might be a bit surprised by the idea the same way my kid was surprised the first time he saw purple M&Ms. But after, like, five seconds it's like, "Oh. So this is the same basic concept as literally any other relationship/M&M I've ever encountered in my young life. Cool. Whatever."

This was perhaps best conveyed by this little chap a few years ago...

Point is, letting your kids know that anyone can fall in love with anyone else doesn't have to be a "very special after school special" conversation "when they're old enough." Anyone who thinks kids can ever be "too young" to be aware of non-heterosexual romantic relationships, by asserting that, is essentially just admitting, "I think there's something wrong and bad and dangerous and upsetting about non-straight people." Do you not feel that way? Then do yourself, your kids, and the world the favor of unburdening yourself of the idea that kids need to be grown up and holding onto something sturdy before they find out that some kids have two mommies. This is something anyone who understands what love and relationships are can understand. This sets the stage for your child to know acceptance is not for a select few, but for everyone.