I just had a conversation with another mom on this very same topic earlier this week. I know that this topic is on the minds of many parents, so I thank you for bringing it to this discussion forum, so that we can learn together how to tackle this very difficult issue.
 
I think that it is important that you keep in mind that you will have many, many conversations with your daughter over the years about this issue, so there is no pressure to "get it right" this time. Your job at this point is to help your daughter understand the importance of respect and tolerance for others, no matter what side of the debate she ultimately will fall on. And, I would also encourage you to share your views with her and help her to understand the way that you see the world and explain that others have differing opinions, so that she will have a foundation for understanding the debate.
 
So, how to begin? I would start by choosing a quiet moment at home or in the car, and say something like, "I noticed recently that you have made comments recently about how weird families with two mommies and two daddies are. Can you tell me more about that?" Spend the first part of this conversation listening and trying to understand her point of view.
 
Depending upon what you learn, your conversation may go in a number of ways, but you will likely want to explain your views. You might say, "I understand that you feel this way, but I want you to know that I feel....[whatever your opinion is]." Reassure her that she won't understand all of what you are discussing.
 
You appreciate how complicated it is and most adults have trouble understanding all nuances of the debate. Let her know that above all, no matter what her opinion on the debate, it is important to be respectful, and encourage her to keep "weird" comments to herself. She wouldn't like it if one of her friends told her that she is "weird" because she doesn't celebrate Hanukkah, or Christmas, or whatever it may be, would she? People's feelings get hurt, and we want to be respectful of other people's feelings.  I have also included links to a couple of articles on the topic below.
 
Best of luck to you.  L. Compian, Ph.D. Counseling Psychologist Education.com

I'm also a straight mom raising a first grader (and a preschooler) in California so I share your pain! I recently had the "joy" of trying to explain to my children why someone would come into our front yard and violently shred our "Vote No on Prop 8" sign. Something I hope I never have to do again.
 
I just keep trying to tell them what their daddy and I believe and promising them that I'll keep doing whatever I can to help make sure the world is a better and more fair place when they grow up and fall in love and want to get married. I also try to encourage them that change is possible by telling them that when their grandparents were kids the "definition of marriage" said that black people and white people weren't allowed to marry each other and that people who realized that was wrong worked and worked and didn't give up until the definition was changed. Today we have to work and work and never give up until we open the hearts and minds of those who think men shouldn't be allowed to marry men and women shouldn't be allowed to marry women.
 
I wouldn't be too hard on your daughter for her recent behavior. It's obvious that you've laid a foundation of tolerance so she's probably just picking up on cues from her peers ("that's weird") or she's just stating a fact that she doesn't even realize is hurtful ("I have a mommy and daddy and they don't"). My four year old recently noted out loud that a woman we walked by on the street was "REALLY fat". He was genuinely confused when I jumped all over him for saying that. To him it was just an honest observation like "the sky is blue". There's an awful lot to learn about what's ok to say out loud and what isn't.
 
Instead I'd just continue to talk to her the way you have been. If you're like me it's really hard to try to explain something you truly don't understand yourself. But I make myself keep trying because I know it's one way to help keep my promise of making the world a better and more fair place when they grow up. They're the future after all!
 
Hang in there!
 
Kat