The marriage institution in most countries puts men at serious disadvantage in case of divorce - which is where a pretty large chunk of marriages end up.
A (free) book on the subject: http://www.realworlddivorce.com/
The other issue, probably more relevant, is that you split all you have in half (roughly). In theory, it is to make up for the fact that the woman has to deal with childbirth and all the consequences, including missed opportunities. In practice it means that even if you don't have any children, you need to give her half of everything if she decides so. Some people consider it very unfair, but of course every situation is different.
The fact is, women file for divorce more often than men, and not everybody is comfortable with a prenuptial agreement arguing that marriage is based on trust, and if you don't have that trust at the beginning, why to marry at all?
EDIT: Theory aside, when you read about particular cases like this one, they are depressingly sobering: https://www.quora.com/Why-do-most-women-have-the-advantage-d...
At least in Germany, AFAIK you only split in half what you earned during the marriage - and that seems fair. Like two founders getting 50% of a startup.
I don't know, none of this strikes me as a big reason not to start a family with a partner that you see eye-to-eye with.
Equal co-parenting is hard, it pretty much requires that you live in or near the same school district as your ex. Being a weekend or summer co-parent doesn't have the same constraints.
It won't be the right solution for every case, but it is always available.
I know several single dads raising kids with little or no co-parent from the mother, but it is the exception, I know far more who went with "every other weekend plus one midweek night"