Pardon me, I must have missed the election that was held to determine who could define who is and is not a conservative. Congratulations on your apparent victory.
Here's a few reasons as to why some conservatives might not be actively resisting gay marriage:
1) That mucking about with the Constitution thing, heavily discussed on other threads, so I'll leave it alone for now.
2) Sizing up the battle, and deciding that the time to have stopped gay marriage with an amendment was during the Reagan or first Bush administrations. Relying on a DOMA signed by Clinton, who promised his friends that it wouldn't mean squat to a SCOTUS that had Ginsburg and Breyer on it, as well as Souter, Kennedy and Stevens, plus whoever he had the chance to appoint in a second term, was not enough. No serious challenge to civil union was mounted after Vermont, and while the Massachusetts Supreme Judicial Court might have surprised some people, it was bound to happen eventually anyway. I don't remember GWB making it a campaign issue, and he certainly didn't rush to put an FMA into law either after he assumed office, or after 9/11, when he was politically stronger.
What resources do we really want to throw into this battle, anyway? It's probably not worth giving up tax cuts, spending increases, or whatever else it would take to add Rat votes to get to a two-thirds majority to move this out of the Congress. Add in the horse trading it would take in many states, and the cost of this becomes enormous, with only thirteen states being able to make it all for naught.
3.) A general unease that the mushy middle has on the subject of gay marriage could resonate with votes in the fall. I'm presuming that MA will do nothing when they get back to work on this in March, after two days of being absolutely unable to come up with an amendment this last week. If, indeed, MA voters are going to punish their legislature for allowing gay marriage to go absolutely unchallenged, they might make MA a more Republican state.
This could also happen throughout the country, as well. It's been pointed out that this will be a big liability for Kerry, who at this point in time, is still the presumptive Rat nominee, and even if he isn't, the actual nominee will not have views anywhere to the right of Kerry on this subject. Why not let the left defend it, while Republicans talk about national defense, and when asked about gay marriage, simply say, we need more people in our party to have a hope of challenging it?
4) There's the freedom issue here, too. You can label all conservatives who don't share your views on homosexuality as libertarians, but it doesn't change the fact that there are people who believe in small government, lower taxes, a strong military, and the right of the people to bear arms, who are not uncomfortable with gay marriage. They know gay people who are not the nasties that ride on floats in gay pride parades, and while they decry promiscuity in both hetero and homosexual relationships, they want to lend their approval for fidelity in the people who society has marginalized. They probably know some gays who share their beliefs in all of the above normally conservative views, who would vote Republican if the gay issues that the Rats support were off the table. I'm pretty sure that the rabidly anti-gay folks here find that hard to believe, but if they had any associates who were gay, those associates would not let them know about it.
5) Perhaps you disagree, but it is not necessary to be a Christian, or an Orthodox Jew to be a conservative. Unencumbered by a strict religious view against homosexuality, these conservatives fear the establishment of a theocracy, or at least having the Republican Party being viewed as wanting to establish theocracy.
In five years, I assure you, we'll all have moved on from this, some of us are just ready to do it now.