I think we all know that there is a time when robots will evolve to the point that they will be able to look and act on a level that they could replace most human functionality. That will with a doubt in most reasonable people's minds lead to a particular sect of society no matter how large, to see these robots as objects that can be used to provide services to humans on every conceivable level. In the beginning this could mean day care or nanny services, helping in nursing homes, you get the point.
If one could make robots that seem to act and feel real, at what point would someone try to give it emotional responses that seemed to pantomime those of humans. I'm not saying they could ever love, but a person could certainly love them.
All ethical dilemmas aside, imagine the long-term benefits of these actions. It could put an end to all prostitution and human trafficking, if robot escorts became cheaper and easier to maintain than their human counter parts, plus with maintenance and clean-up it could also take away any risk of STDs which would have no chance of being able to survive on any synthetic skin.
Would congress pass laws forbidding the use of robots for any and all sexual use? Would the people that did these actions be shunned or mocked by the majority of society? Would their be robot advocates that would speak out in favor of robot rights?
Oddly enough, in all of Assimov's books, I don't think he ever tackled the idea of human/robot sexual encounters. I'm left only with shoddy examples like the movie AI.