At first when I read this, I immediately thought that his behavior was out of character due to alcohol consumption.

But later upon hearing you state that you both were sober enough to understand the circumstances, there really is no explaination. You also mentioned that you had only slept with him once over the course of five years. How was he the first time? What made him so bold and uncoperative with you this time? I'm willing to bet his assertiveness comes out in normal behaviors towards you and others as well. I'd be shocked if I was wrong.