You seem to be identify a valid problem of defining "assault". This is a real problem, and something that policy makers will need to tackle at all levels, but it is not what is being discussed. After someone responds to your request to sleep with them by asking you to leave, you have left the realm of ambiguity.

If you are hanging out with a friend and start getting a bit handsy, then I could see the grey area. But it the cases where it is not wanted, a simple "no", or movement away, or pushing his hands off, etc would be enough to clarify where the line is, and ignoring that would turn it into unambiguous assault.