I'm pretty sure it's some combination of the decisions of both parties making the agreement, and the relevant laws.
Upvoted because I can't see any reason THIS comment should be negative, even if I STRONGLY disagree with the "just assume they apply" comment beforehand.
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"?