Yep. To put it another way, take the question "to whom should X be married?" If the answer is "it's 100% personal preference - no choice is better than another"

Yep. To put it another way, take the question "to whom should X be married?" If the answer is "it's 100% personal preference - no choice is better than another" - then the state shouldn't get involved. We don't pass laws encouraging you to wear blue instead of green, because it's just a matter of preference. So if your marriage choices don't matter to anyone, why should society bother recognizing and investing in them?