Acausal marriage is a great way to lower your acausal taxes. Of course, most people don't actually pay their acausal taxes anyway.
Not to mention, there's no law against acausal same-sex marriage. I guess there might be an acausal law against it, though. It's kinda hard to tell.
I just realized I haven't previously pointed the metaphysicians on Less Wrong to "Possible Girls," a hilarious paper about acausal romance:
Even if you don't read the whole thing, don't miss the final paragraph.