What are the best arguments you have in favor of marriage?
For some people it's a commitment that allows them to trust their partner more as they've both at least claimed that they will stand by each other through thick and thin. For others I know they think it's outdated and obsolete and a horrible institution and IMO, most of those folks can trace back to a negative event or person that caused them to hold their current view.
Ummmmmm, gimme a while and maybe I can come up with something.
Best way by far to bring up kids is with both parents together, marriage strengthens a couple, or should. But few take the vow seriously. I you don't get on, don't have kids, once you do have kids you owe it to them for you both to get on.
Societal pressure if you have kids. Not an issue in some areas.