Why does it matter who initiated it? The correct answer either way is To not get sexually involved with subordinates. There is no moral ambiguity here. It is wrong to abuse a position of power.
If you reason for not getting sexually involved with a subordinate is that it is wrong to abuse a position of power, how is there not an ambiguity there - or the rule goes out the window - if the subordinate is initiating?
To pretend that the "correct answer" is always so straightforward is to assume that people never make it messy by addressing your assumptions and actively trying to change your mind about it.
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.
