Like I said, YMMV. This is not an assumption so much as an observation of dozens of couples in various industries, with both men and women in the superior position. This isn't necessarily a conscious motivation on the part of the subordinate, but it exists. Like other temporary motivations (e.g. beauty, wealth, vigor, etc.) this one sets up the relationship for a transition when the motivator no longer exists. Many relationships survive such transitions; many do not.