It might be that a person with roommates is more likely to be introduced to more people and therefore more likely to find someone to marry, than a person without roommates. No idea if there is any data on that.

My own experience is that when I had roommates they would invite people over or invite me to activities where I'd meet new people with very little effort on my part. Vs when I've lived alone it felt less far less likely to meet new people without more effort.