There are many reason why someone would be a pain to work with honestly. I could imagine someone insisting on people going to drink after work being a pita for a muslim for example, or a guy talking all the time about politics and having different views than myself, etc, etc. But it seems like sex is handled extremely differently than the rest, and i'd like to understand if, in practice, there are reason for that (other than the subject itself being extremely sensitive in the US compared to some european countries, for example).
"sex is handled extremely differently than the rest ... [is there] reason for that[?]" I mean US internet culture is the place where this baleful notion of the "friend zone" exists: men who complain bitterly that being nice to women does not secure for them a sexual relationship. Denigration or retaliation following sexual rejection is a well-documented practice. From what history do you think sexual harassment laws spring? Women fought for them because they survived regular abuse, manipulation and retaliation from men (especially superiors). Have you read any of this history? for example: http://www.csmonitor.com/USA/Society/2011/1114/The-evolution...
It says something about the culture of the company that a manager feels unfettered to ask his direct reports for sex so boldly, i.e. without fear of consequence. I'm shocked you do not see this.
There's a power relationship between reports. Most companies have explicit guidelines that prohibit romantic relationships along the management chain.