The problem is that vulnerability is a feeling state. A different person in the same circumstances could feel more or less vulnerable or not vulnerable at all. When we add private (no witnesses) sexual relations to the mix and the halting conversation that happens between people who are starting to have them, it's possible that two parties may have wildly different views of what is happening.

That's a very dangerous place for law and social retribution to go. It would actually be better to outlaw sexual relations between people in reporting relationships in employment.