Time-limited marriages are a thing. An old Islamic thing http://en.wikipedia.org/wiki/Nikah_mut%E2%80%98ah and something you see pop up sometimes as a suggestion like http://abcnews.go.com/blogs/headlines/2011/09/mexico-city-considers-temporary-marriage-licenses/
Recently, LWers Will Ryan and Divia Melwani (now Will and Divia Eden) were married, with Eliezer Yudkowsky officiating.
I've been to 40+ weddings in my lifetime, and this was my favorite ceremony yet. Here is the video, and below is the transcript of Eliezer's... what's it called? "Blessing"?