Like all moral questions, at its base it's really aesthetics or even "taste". Some managers are not attracted to those who are willing to exchange romance for career advancement. (...they got a name for that.) Other managers are attracted to that sort of person. The former group of managers has less difficulty with this situation than the latter group has, so one might say they have better taste; YMMV.
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.
