Because it isn't true, no one cares about whatever your personal differences with libertarianism are, it doesn't change libertarianism.
Look at this Cato article for instance and how libertarianism is defined. It is defined just as I always define, conservative economics and social liberalism. """We define libertarians as those who favor "smaller government" and think government should not promote "traditional values."""
ie ""Sexual orientation, preference, gender, or gender identity should have no impact on the government’s treatment of individuals, such as in current marriage, child custody, adoption, immigration or military service laws.""
There are tons of definitions of small l libertarians, anyone who understands it knows that it includes social conservatives and social liberals - and either way they don’t believe in gov’t force to do their dirty work.
A legit small l libertarian is never gonna advocate having a gov’t force Christians to accept a definition of marriage they do not feel is Biblically appropriate. Anyone who says otherwise has been warped on what small l libertarianism actually is. Those who know can clearly tell you that a true small l libertarian would be completely revolted at forcing churches to perform gay marriage ceremonies against their will.