Marriage collapses two separate legal entities into a single entity, putting you even more at the mercy of the increasingly extractive financial/legal system. In a committed relationship, how does it make sense to throw away the flexibility of having independent legal entities for each of you, especially when creating new solid legal entities is quite expensive ?
With everything being increasingly financialized (eg the medical billing industry cancer), there are many sources of extreme liability these days. Long tail events that can stick you with some insane amount of debt that will drastically alter the course of your life. If we're married, any debts pinned on my partner automatically end up being my responsibility, meaning our entire life is completely at risk.
Whereas if we're not married, then each one of us is effectively holding half of our wealth in an informal trust for both of us. And for solidifying and committing to that arrangement, it would seem these trusts could be formalized in a way the courts would have to respect. Rather than defaulting to the expected traditional legal transaction, becoming a singular entity, and then having to grasp at sketchy asset protection methods and the like.