To me, it seems pretty obvious. The viewers are being offered the idea that Ennis truly was a gay man, and in love with Jack for life. Here a gorgeous, sexy, fun girl, in short, a straight man's ultimate fantasy, presents herself to Ennis on a silver platter, yet, despite her willingness and her candour at making the first move (meaning there is little more for Ennis to do other than accept), he STILL cannot commit himself to a woman.
Maybe he realizes on some subconscious level (I tend to believe all of Ennis's realizations are subconscious!) that he simply cannot enter another heterosexual behaviour and hurt himself and the woman in the way he hurt his own marriage.
In my opinion, we are being told that while Ennis has the ability for women to fall in love with him, he simply is incapable of loving a woman truly and completely, because of his own sexuality and because of Jack Twist.