Who marries who?

Jordan's classmate Isabella expressed a wish to have a playdate with Jordan, which is rather novel because she is a girl. At dinner last night, I asked Jordan about Isabella, and we had something like the following conversation:

Jordan: She's okay. But I don't want to marry her.
Rachel: Isn't there another girl that you were going to marry? Anna? [There is a lot of marriage talk among the kindergarteners... I have been taking it as just statements of affiliation and going along.]
Jordan: She wanted to marry me. But I didn't want to marry her.
Rachel: Why not?
Jordan: She does not have a nice voice. [I found out later that she allegedly yells 'Shut up!' at people, so, that makes sense.]
Jordan continued: I would want to marry Sonya, but she wants to marry someone else.
Rachel: Who does she want to marry?
Sonya: Her little brother.
Dale: Oh, well, actually she can't do that. You can't marry your little brother.
Jordan: Why not?
[Dale tried to explain about what happens if siblings have children and quickly ran aground. Not only is it hard to explain, it was hard to even explain why that had anything to do with it. And is it really the issue, after all? or is it just a rule? This led to:]
Jordan: What is the rule about who can get married? Can a girl only marry a boy and a boy only marry a girl?
Dale: Well, at this time, in this state, that is still the rule.
Jordan: But Sonya can't marry her brother?
Rachel: You can't marry anyone in your family. That's the rule.
Jordan: So who can marry who?

Instead of just stating the law, it seemed like a good time to say what marriage means - what people are choosing, when they choose to get married (or want to). This is a contested issue to say the least, but as parents we get these little opportunities to say what we think it means, so, bombs away. But what is my ideal? It definitely does not hinge on making or raising children. I tried "You marry someone when you want to live together forever." It's often not forever... but maybe you want it to be? Must they live together (or want to)? Dale didn't go for it. So I tried, "You marry someone when you to make him or her your family." This is not only pretty darned inclusive, but also has the advantage of explaining why you can't marry your brother - he's already your family.

I wonder what the norms will be for Jordan's generation?