I think I would avoid describing any social distinction as, "the default," to children. Do you tell them people are poor or rich by default? Happy or sad by default? The whole dilemma is predicated on looking for a world view that fits into simple binaries.
I would tell children what marriage is, as an idea, and when they ask who can get married you can say, "adults, or children with their parent's permission." It seems like you're concerned with fostering bias or bigotry in children. You will, we all will - our children will generally inherit our prejudices and politics. Part of raising children is working to create an environment where they understand how you feel, but also that disagreement is ok and questioning is ok.
I would tell children what marriage is, as an idea, and when they ask who can get married you can say, "adults, or children with their parent's permission." It seems like you're concerned with fostering bias or bigotry in children. You will, we all will - our children will generally inherit our prejudices and politics. Part of raising children is working to create an environment where they understand how you feel, but also that disagreement is ok and questioning is ok.