When it comes to children they don't understand either. They know mommy and daddy are married, they know grandpa and grandma are married but what they don't understand is how aunt Kim and aunt Cheryl can get married. At their age they get confused and require an explanation of what is going on. I know already been there with Aunt Kim and Aunt Cheryl, hence the use of these names.
We've had a few conversations about this here, the longest one was probably the one about "Postcards from Buster." My daughter has been "getting" this stuff with no problem at all from when she was very small. Probably in no small part BECAUSE it started when she was very small. As ebrown says, heterosexual marriage is everywhere -- the "happily ever after" of bedtime tales from the very beginning. If that's all a child sees, then something different would be remarked upon, sure.
If a child is aware from very early on that sometimes men and women get married and sometimes women marry each other and sometimes men marry each other, it just isn't that remarkable.
50 years ago, it was unusual for black people and white people to get married. A child may have required explanation if a interracial marriage did occur. That didn't make the interracial marriage wrong.