Relationships have a far bigger economic basis than most people want to admit. In many ways, a relationship is like an employee-compensation package. Even if the salary is on the low side, you might stay because the benefits are good, you get along with your boss and a short commute time (or you get to work from home). If your employer starts to shaft you, you start looking for another job. Likewise, if you can't provide for your woman, she will start looking for a better deal and cash out when she finds one.
Sex and attraction are part of the equation, but far from the only thing. It's also much more important early on. It's like the support that holds things together while the concrete hardens. Children also bind you together.
After enough time has passed (20, 25, 30 years), the sex aspect has probably played out, especially on her side. Her ability to leverage sex is just about gone. At that point, It's just two companions who have a shared history and take care of each other. And, of course, if you divorce, she takes you to the cleaners.