What I find fascinating is that the males didn't choose to directly sexually assault a woman. They didn't choose to involve their own bodies.
I can think of two reasons for this:
a. The sexual thrill was in watching - i.e. it is voyeurism similar to other kinds of voyeurism. This seems the less likely option.
b. The thrill is only indirectly sexual - instead it is all about power and domination, the thrill of abusing a woman. The dynamic is like most rapes, in that it is about power and violence, not sexual gratification. They could have got the robot to simply push or punch the woman, but it had to be a sexual action to drive home that this wasn't about any old violence towards any human, it was specifically about male domination over females.