Ben: I probably won't be able to marry Elissa, because she's a baby.
Me: She won't always be a baby, but you can't marry your sister anyway.
Ben: (Incredulous) Why can't I marry my sister?
Me: (Thinking very fast, babbling incoherently and then settling on) Because if brothers and sisters have babies, the babies can be sick. (There was some talk about getting some stuff from both parents and having similar stuff yada yada, but without any concept of recessive genes, it wasn't exactly intelligible)
Ben: But I couldn't marry a school kid
Me: Yes, you could, when you are older
Ben: But I couldn't marry my favourite
(some failed guesses)
Me: Oh, you mean [insert bestie's (male) name]? Well, yes, you could if you wanted. (Ignoring the subtleties of legal marriage not being an option)
Ben: But he's not a girl!
Me: Sometimes boys marry other boys.
Ben: But how could they have babies?
Me: Well they can't.(Ignoring lots of other subtleties that are probably a bit much for a 5yr old)
Ben: It would be really funny if two girls got married.
Me: That happens too.
Ben: WHAT???? But what if they both had babies at the same time?
Me: Well you need a boy and a girl to make a baby, so they would need help from a boy. But mostly what would happen is chaos. Two babies to look after!
Ben: But what about twins?
Me: Yes, twins are chaos too.
Ben: Why can't we have twins?
Me: Because they happen by accident, you can't just choose them, because I don't really want them, and because we aren't having any more babies!
Well, that covered some ground in 5 minutes. :)