I have never explained it to my kids. They've never asked me any questions about it. They don't think of it as something that needs to be asked about, because they've always been around queer people, it isn't something new and strange being introduced.
I only brought it up with mine because some kid with backwards ass parents in her kindergarten class was saying that families can only have a mommy and a daddy. My wife and I said, "no that's wrong" and the child said "ok" and went about her day! They grasp this SO easily!
I remember when my kids saw two women kiss for the first time on SUPERGIRL. They were like “that can happen?” I said yes. And we just kept watching.