I work in an extremely liberal environment and have come to understand how "progressives" approach their causes. My experience is that they are intolerant of people who do not share their convictions--more so than most Americans.
One might think that legalizing gay marriage in all 50 states would put that issue to rest. Such would not be the case for the gays. The problem would be that although men could marry men and women could marry women--there would be people who still would not accept or approve of it--possibly as many as half of the population. Whether it be for religious reasons or just that it's weird, people would still be critical of gays being married. This is totally unacceptable to the gays.
You see, although they can consider Conservatives "Teabaggers" and Christians "Bible Thumpers," that's okay. But they would demand that married gays not be treated with the same disdain. Afterall, they're gay and we have to be "inclusive" (even if they don't see that as applying to themselves). Everything from new laws to public humiliation would be used to get us all on board with the "naturalness" of gay marriage.