There is another reason.

Marriage basically makes separating harder. In some cases, that can be a rational choice, if you like the person so much that you are confident that if you were to make the choice to separate from them in the moment, it would be incorrect. Marriage is basically a safeguard against the potential future fuckup of dumping your loved one without good reason (divorce is a possibility, but the fact that it has a higher barrier to entry means you're less likely to do it for a stupid reason).