You can have kids with a partner, but really no reason to formalize it with “marriage”. I guess there are tax reasons though that make it more logical though.
But if nothing else, marriage is an agreement that you’re not just gonna nope out when there’s adversity or when something better comes along. It’s a powerful mutual gesture.
If you aren’t compelled by that idea, there’s someone out there who will. Maybe you have to dump your current 6-year-long situationship to go look for them, but hey, it’s not like you’re married. ;)