You are right. I guess because this sort of crosses the line into "rape" and prostitution. Expectation to have sex on the job sort of straddles those lines a bit, but I see your point.

Let's say there weren't any moral hang ups and people readily put this thing in job descriptions. If you refused when offered then it's fair game if you're fired.

I have no exact answer here, but I do empathize with your angle.