This is where libertarianism veers away from conservatism.
The basic assumption difference is that conservatives view our culture as something worth preserving,
while the libertarian sees nothing special about our culture and would let it deteriorate and die.
The libertarian has a blindspot where he doesn’t see that a “moral and religious people” is NECESSARY for the liberty that he sets as the highest goal.
Or it could just be that sanctioning marriage isn't one of the clearly defined powers granted to the fedgov in Article 1, Section 8. But far be it for me to throw that in the way of a good demonizing of libertarians.