it seems implausible that we'll ever have a definitive list of all types of consensual human sexuality

A point which begs the question of why the government should be involved in the first place. Really, and I think most of us would agree here, the government probably has no business governing people’s sexuality. If we lived in religiously controlled countries it might make more sense because we’d be adhering to an explicit religious law, but the law of a religiously neutral country shouldn’t tell people what they must/must not/can do in their sexual lives.

So why the concept of “marriage” at all at the governmental level? It seems to me that all of the laws and issues regarding what the state uses “marriage” can be more efficiently addressed by properly identifying the problems (can you tell I’m a software developer?). If child rearing is the issue, then offer tax breaks for primary child care givers, whether married or not. If it's a visitation or asset ownership issue, let a person designate whoever they want to fulfill those roles. Not that I'm offereing soltuions or have properly identified the problems, but my point is that identifying each issue individually is probably more constructive than trying to fit it into the concept of a "consensual sexual relationship." I think marriage has been used because in the past it roughly corresponded to the way these needs existed in the real word; however as society changes this concept should be exploded and the real issue addressed. Let marriage be whatever people want it to be: those in my church can define marriage as between a single man and woman without being confused with any legal entity of the same name. In Utah the church can have its own version of marriage with multiple partners. Certainly each church will feel that the other churches are fostering unhealthy relationships, but at least the concept won’t be confused with a legal construction that has nothing to do with the church.

I know, I know . . . it's not going to happen, but that's what I think anyway.