Here's a place where I feel like my models of romantic relationships are missing something, and I'd be interested to hear peoples' takes on what it might be.

Background claim: a majority of long-term monogamous, hetero relationships are sexually unsatisfying for the man after a decade or so. Evidence: Aella's data here and here are the most legible sources I have on hand; they tell a pretty clear story where sexual satisfaction is basically binary, and a bit more than half of men are unsatisfied in relationships of 10 years (and it keeps getting worse from there). This also fits with my general models of mating markets: women usually find the large majority of men sexually unattractive, most women eventually settle on a guy they don't find all that sexually attractive, so it should not be surprising if that relationship ends up with very little sex after a few years.

What doesn't make sense under my current models is why so many of these relationships persist. Why don't the men in question just leave? Obviously they might not have better relationship prospects, but they could just not have any relationship. The central question which my models don't have a compelling answer to is: what is making these relationships net positive value for the men, relative to not having a romantic relationship at all?

Some obvious candidate answers:

• Kids. This one makes sense for those raising kids, but what about everyone else? Especially as fertility goes down.
• The wide tail. There's plenty of cases which make sense which are individually unusual - e.g. my own parents are business partners. Maybe in aggregate all these unusual cases account for the bulk.
• Loneliness. Maybe most of these guys have no one else close in their life. In this case, they'd plausibly be better off if they took the effort they invested in their romantic life and redirected to friendships (probably mostly with other guys), but there's a lot of activation energy blocking that change.
• Their romantic partner offering lots of value in other ways. I'm skeptical of this one because female partners are typically notoriously high maintenance in money, attention, and emotional labor. Sure, she might be great in a lot of ways, but it's hard for that to add up enough to outweigh the usual costs.
• Wanting a dependent. Lots of men are pretty insecure, and having a dependent to provide for makes them feel better about themselves. This also flips the previous objection: high maintenance can be a plus if it makes a guy feel wanted/useful/valuable.
• Social pressure/commitment/etc making the man stick around even though the relationship is not net positive for him.
• The couple are de-facto close mostly-platonic friends, and the man wants to keep that friendship.

I'm interested in both actual data and anecdata. What am I missing here? What available evidence points strongly to some of these over others?

Edit-to-add: apparently lots of people are disagreeing with this, but I don't know what specifically you all are disagreeing with, it would be much more helpful to at least highlight some specific sentence or leave a comment or something.