Yeah, thanks. Me and my sister have loads of sex, as it happens. In fact, we have four kids with downs. We're not married, though.
Edit: My point, in case it was lost in a torrent of vomit, was that the link between children and marriage has been more or less confined to the dustbin of history. Furthermore, it raises slightly worrying questions about coupled who aren't related but who both have inheritable traits that have a strong chance of leading to conditions. I don't think many would suggest they be banned from marriage (or even having kids), though many would warn them against it and perhaps point them towards adoption. Inbreeding is a pretty poor argument against incestral marriage.
Edit: I'm doubling down. A second edit! O! Here's a quote from everyone's favourite American substance abuser Doug Stanhope:
"If marriage didnt exist, would you invent it? Would you go Baby, this shit we got together, its so good we gotta get the government in on this shit. We cant just share this commitment tweenst us. We need judges and lawyers involved in this shit, baby. Its hot!"