The Never Married, a New Normal

I think it's a lot more complicated than a simple genetic trait: people weigh the cost and benefits using a lot of no-single-gene high-level reasoning and (in this environment, the massive inequality in haves and have-nots, the relative lack of material support for raising a child from the larger community, the overall ease relative to decades past of access to the tools of controlling pregnancy, and the removal of most of the patriarchal obstacles to women in particular remaining marriage-free) decide that it's not worth the cost. It's not like there's a risk of failure to propagate the species; if anything, the risks seem most associated with increasing the headcount, not decreasing it.

In the US at least, being raise by a single parent is a massive statistical indicator for basically every negative social outcome. Poverty, criminality, future generational single parenthood, etc.

Divorce isn't the same thing as "losing big". The latter is a relatively rare occurrence, which can happen when the relationship is very one-sided (one spouse makes 10x the income of the other spouse), and one spouse is extremely mean and vindictive. Plenty of couples separate amicably. Plenty of couples separate and it's a long painful affair, but no one gets robbed or cheated in the process. If you really find yourself that worried about your wealth and find yourself that worried about getting cheated in a divorce, sign a pre-nuptial agreement. Most reasonable people will accept one.

My point, moreover, is that the risk itself, the state of vulnerability, is something a lot of people want in a romantic relationship. People like you and the OP of the post I replied to see this risk as pure downside. You think that everyone sees the risk this way, and that they only accept it because they think the potential winnings are worth it. Marriage, as you put it, is a huge gamble. But this is not the way everyone sees marriage. Achieving this level of intimacy with a person, where you inevitably have to trust them like this, is itself the goal. This is the "something valuable" I refer to. It means the downside (potentially getting robbed in divorce court) is offset not just by the upside (a good marriage) but also by the inherent value of the risk-taking behavior that marriage involves.

> 1-2 years isn't really enough time to get to know someone well enough to determine if the relationship will work out

Sure! I can agree with that (with the above note that getting divorced is not really the end of the world). So take 4 or 5 years. Live together for a while. Get a shared bank account when you feel you're ready for that and need to pay some bills together. You may even find that you achieve the kind of intimacy you're looking for without going up to an altar and putting a ring on their finger. I've been with my partner for over a decade, and we're not "married". I'm not old fashioned, and this isn't the 1950s anymore. But I am attached to the rather romantic notion that intimacy is inherently valuable, and that it inherently involves taking a risk.

There's lots of reasons.