Scene - a long haired brunette in a long robe stands before a man tied to a chair. She drops the robe and there is full length rear nudity. She straddles the man and tells him she will kill him when he orgasms. She then proceeds to have sex while he tries to restrain himself.
That's all I saw. Would like to know how it ends.

It's the indie thriller Thursday (http://www.imdb.com/title/tt0124901/), the man is Thomas Jane, the woman is Paulina Porizkova.