The power imbalance doesn't just affect the couple in the romantic (or lustful) entanglement. It affect those outside of the entanglement as well.

At the worst it can be akin to nepotism which can quickly erode morale and productivity -- (there's no reason to work harder since the "fiance" will get the promotion).

At a previous company a manager dated her direct employee, and working in her team was really an awful experience because of it. Everyone knew it, and nobody could say anything for fear of retribution.