If consent to sex is given moment to moment, rather than once and forever, then marriage cannot be a durable contract
Not technically true. Since you have already said marriage is being redefined it just means that the redefinition must be to something which does not necessarily include sex---that is, a contract that allows enforced abstinence. A logically coherent concept even though I find the notion repugnant.
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/