One of the interesting things about the recent Supreme Court decision on the Texas anti-Sodomy law was the way in which it went out of its way to strengthen the underlying principles of Roe vs. Wade, while at the same time explicitly rejecting the possibility that the decision would have any effect on same-sex marriage. Soon afterwards, Senate Majority leader Bill Frist endorsed a proposed constitutional ban on homosexual unions.
I think Calpundit is correct to assert that it would probably be political suicide for either party to make too big an issue of this. But I think the real question is why they are bringing it up in the first place? Perhaps because it would be even more suicidal to focus on the implications for Roe vs. Wade? Have liberals already won the culture wars? The demographics certainly seem to suggest that this is the case.
However, the inability to fundamentally change the laws regarding same-sex marriage suggest that a powerful but vocal and politically powerful minority can still block real reforms. I agree with Mindles Dreck that we need to tackle this from the point of view of reducing the importance of straight-marriage rather than wasting our breath fighting for same-sex marriage. As of 2000, only 11% of unmarried partners were same-sex couples. That is 9.7 million Americans living with unmarried different-sex partners! The “Alternatives to Marriage Project” has lots more information and statistics on this. I can’t find links right now (please post below if you have some) but I believe that many Western European countries have already moved in this direction, and more and more young people are choosing to live together without getting married.