Putatively fighting for equality is always an easy way to obtain cheap grace.
Additionally, though, I suspect it is because the same-sex marriage crusade feels purer than other potential civil rights crusades do. A black guy who hasn't accomplished much of anything has been twice elected to the presidency in large part because he is black. Affirmative action and racial quotas give non-whites all kinds of special privileges that result in, ceteris paribus, NAMs being favored over whites of equal ability. Anyone who has spent any time in the corporate world knows that rather than there being a glass ceiling keeping minorities and women down, managers fall all over themselves in an effort to lift up and promote people who will make the workforce more demographically diverse. The contemporary struggle for gay rights, in contrast, is perceived as truly being about egalitarianism.
Perhaps at some point in the future SWPLs will begin to have some misgivings about having possibly pushed the gay rights ratchet too far as well, but we haven't reached that point yet. The pickings are slimmer than they were half a century ago when it comes to finding victim groups to shed a tear for, but the tree isn't bare yet.