Jonah, you’ve never heard a libertarian saying that tax breaks should be extended as widely as possible?
More seriously, the difference between the libertarian case and the examples you mention is likely to be the question of when setting boundaries sinks into, for want of a better pejorative, ‘discrimination’. Let’s look at social security.The libertarian might be opposed to it in principle, but, given that it exists, I doubt that he would support restricting its bounty to, say, women alone. At the same time, he could quite plausibly make the case that, if there is going to be social security, it should be confined to those aged 70 and above. Following this logic, the libertarian argument would be that the decision to restrict marriage to heterosexual couples was an act of discrimination rather than something with (to him) rather more respectable intellectual foundations.