> Another way of looking at it is that long-term relationships with prostitutes would fit your description.
There's not much data on that but I remember vividly reading an academic study that interviewed many prostitutes and cases where regular "clients" used their services for more than 2 decades while racking several divorces in that time where common.
I was really surprised at the time.
Maybe friends disappearing (like the article states) leads those men (in that study it was only male "clients") to pay to have a permanent human element in their lives.
You'd be surprised, but the qualifier "romantic" messes up with clearly addressing this (since it presupposes sexuality).
Marriage in most cultures have been more about the economic and companionship part than about sexuality up until recently. Heck, marriages were mostly arranged in most cultures until the last 1-2 centuries...
After all you can "play" around without getting married too, no need to marry for that.
Sex is great and valuable; it's probably necessary at certain points in the relationship; but it's certainly not sufficient or the most important thing. Love, intimacy and human bonding are far more rewarding and valuable (and all enhanced by positive sex). As another easy example, beyond a doubt anyone I know with kids would give up sex before they gave up their child; it wouldn't be a close decision - it would be offensive to suggest otherwise to them.
EDIT: I omitted a very valuable reason for long term relationships: To have someone to care for, for whom you matter. I believe research (and plenty of anecdotal examples) shows that, lacking this benefit, isolated people feel like they don't really exist.