This thread caused quite a deep discussion:
There are infinite variables and each relationship and person is unique.
Factors:
1. Why is the couple together? Companionship, to have a family, financial security, etc...?
2. Why did the couple choose each other vs. others? "Chemistry", meets their needs, status, etc...

I could site all the different relationships I know of. There are just so many variables. For one example, I know a female doctor who married a man who stayed at home raising their children.