there are a lot of potential negatives [...]
  Why in the world is tying all of your assets
  [...] a thing?

This started making much more sense to me when I noticed that, among my friends, 95% of people who get married have a baby within 12 months.

Once you think of "get married and have a baby" as a single atomic decision, all the "downsides" of marriage are overshadowed by the _inherent_ downsides to having children.

Marriages are difficult to get out of? Well, children are almost impossible to dispose of. Marriages can be personally costly? Children will take up a lot of time and money, no matter how rich you are. Marriages can involve one partner shouldering more than half the burden? Well, a man can't breastfeed a newborn at 4am no matter how much he wants to. And so on.

  pitching the idea of marriage minus mixing
  finances, [...] resulted in hours of crying.

The discussion might go more productively if you approach it from a different angle: Start by deciding if you want children together; then discuss the things that follow from that.