It is important to read body language. It is also important for all parties to speak up if they are uncomfortable. I still believe asking for permission is ideal and not a turn off at all. And I do think my analogy works: I generally cannot just slap guys without asking for permission first. However, I’ve learned I can slap lightly and judge the reaction — this sort of feeling-it-out methodology is acceptable. Going full speed ahead is not.
I would probably not like a guy to say, “Shut up and fuck me,” unless we had talked about me liking him to talk to that way. Otherwise it is presumptive and disrespectful. It makes me uncomfortable if guys start doing things I like without me telling them I like that sort of thing first, if they are at all aggressive. It shows more respect to ask.