I think if you view marriage as "just sitting around," this is not the type of romantic relationship that describes what typical founders likely seek out.

To answer your point more directly, one difference is that a marriage tends to produce children in a way that platonic friendships tend not do. My sense is that equal division of significant income and equity post-marriage will help, not hurt, children.

To answer your point a different way, the default marital property settings tend to work well for most people. Of course you can change those defaults to some extent with prenups, postnups, trusts, and so on. If you're still dissatisfied with your ability to alter those defaults, well, nobody's forcing you at gunpoint to marry someone you like to live with, have sex with, and even raise kids with.

Because that's what marriage is. If you don't like it, there's an easy solution - don't marry. You can do practically everything - from sex to going to movies together to emotionally supporting - without being married. If the concept of sharing your life with another person sounds weird - just don't do it. It's not mandatory, you know.

No, there are different kinds of marriages. One can have an agreement that past and future assets are not divided. If you don't like it, don't get such an agreement.

>If the concept of sharing your life with another person sounds weird - just don't do it.

Sharing your life with another person doesn't imply sharing property.

Careful about community property/common law marriage, though. Most of the liabilities, few of the benefits.