"They never lobby for (or introduce, if they're in a position to do so) bills to actually achieve that end, and in fact usually do nothing and let the status quo -- which until very recently was "keep the discriminatory laws on the books" -- stand."
How many self-professed libertarians are there in the Congress? Paul's not even self-professed libertarian. The only one I know is Justin Amash, and he couldn't introduce any of type of bills you reference because under Boehner's regime only bills blessed by his team could ever make it to the floor.
Very true—what libertarians should do is promote common‐law marriage, which is permitted in 9 states. Unfortunately, over time fewer and fewer states have allowed forming new common‐law marriages. And in those states where it’s still possible, same‐sex common‐law marriage has always been illegal. So “get the government out of the marriage business” still requires active effort to change the laws.
https://en.m.wikipedia.org/wiki/Common-law_marriage_in_the_U...
So in extensional terms, such "libertarians" are often indistinguishable from theocrats on key social issues (i.e., regardless of what different set of inner philosophical principles they might claim to be following, the outward result in terms of lawmaking is identical). Which in turn means they're not going to get my vote.