But as Saint Paul rather delicately said, and people in the eighteenth century rather more plainly said, enforced abstinence is not going to fly.
Yeah, in certain circumstances people are going to have incentives to break promises (and/or contracts). I don't think that this is specific to marriage, and I don't think it makes the concept of marriage invalid.
You cannot, or at least should not, ask people to contract to that which they cannot perform. Thus, moment to moment consent to sex, requires in practice moment to moment consent to marriage, which abolishes marriage. Abolishing marriage violates freedom of contract.
Which is not moral progress.
Yesterday I sat down with Lukeprog for a few hours and we produced this ten-page interview for the Singularity Institute blog. This interview contains information about the Singularity Institute's technical research program and recent staff changes that hasn't been announced anywhere else! We hope you find it informative.
http://singinst.org/blog/2011/09/15/interview-with-new-singularity-institute-research-fellow-luke-muehlhuaser-september-2011/