I wanted to take a moment to respond to Jonathan's argument below, but I didn't want to stick it in the comments.  First, Rod's point in the first comment is right on target - why should anyone who self-describes as a libertarian (whether as a pure noun or as an adjective modifying conservative) care how the state defines marriage?  If the recognition of same-sex relationships is the right thing to do, it's the right thing to do.

Jonathan also argues: 

First, the people proposing the same-sex model as an analogue to heterosexual marriage don't normally respect the model (if they did, our welfare state would be much smaller!).

As one of those not only proposing but supporting the same-sex "model" as not just an "analogue" to heterosexual marriage but as a legitimate form of marriage itself, and as a flaming heterosexual, happily married for almost 16 years with two children, and none of us being on welfare, I find that argument more than a little bit problematic.  There are many of us out there who believe in the importance and value of heterosexual marriage, and who practice it, and that's precisely why we want to open the institution up to those who wish to, but LEGALLY cannot, participate in it.  (Society has opened the institution up before, after all.)

But the big point here is that Jonathan's arguments about the problems involved in non-marital same-sex relationships (how long does an affadavit last? the poor incentives to report relationships accurately, etc.) just prove too much.  These are precisely the reasons to expand the institution of marriage to same-sex couples.  Yes, the state of Illinois hasn't done so, but institutions such as firms and universities all across the country have extended benefits to same-sex partners and no one's turned into a pillar of salt yet, nor have fake same-sex relationshps driven up heath insurance premiums everywhere.  We're not talking about a lot of people here, and organizations can find relatively non-intrusive ways of making the world of the second or third best work.  Of course, just opening up marriage would solve all of these problems (and a few others as well).  Or getting the state out of marriage altogether....

Given the state's involvment, however, Jonathan is right in raising the question of why heterosexual non-married cohabitants can't also get a piece of the benefits for cohabitants action.  Why exclude them?  But this too proves too much.  As Jonathan Rauch and others have long argued, arrangements for same-sex couples short of full marriage will have a difficult time excluding heterosexuals, and in the process, will undermine heterosexual marriage.  If you believe, as I do, that marriage is a desirable social institution, then why not both extend it to others who wish to participate in it and avoid undermining it with what Rauch calls "marriage lite?" 

For those of us who think that including same-sex couples in the institution of marriage is both a matter of justice and something with net positive social benefits, the move to extend employment benefits to same-sex couples by firms, universities, non-profits, etc., is one very good way to move toward that goal.  It's bottom up, decentralized, and responding to local preferences.  I would argue that the increased support for same-sex marriage over the last decade has to some degree been the result of the increasing recognition and inclusion of same-sex couples in civil society in just these sorts of ways.  Frankly, I applaud your university for moving in this direction, and I think it's one that libertarians should support - again, at least as the right thing to do in the world of the second best.