Because she's not actually able to properly consent if she feels her job is on the line, whether it actually is or not. When a supervisor comes onto their subordinate, that subordinate may feel that they'd be putting future work opportunities (e.g. promotions) in jeopardy if they say no. Or worse, they'd worry that they're putting their employment at risk. Even if Jensen had "pure" intentions and this affair in principle didn't affect their working relationship, he in fact can't know that.