What's funny is that in the comic itself Marten and Claire are a happy couple, so presumably their sex together was highly enjoyable. And, presumably, Marten's expression in that last panel is meant to be one of happiness, of arousal.
It's just that Jacques is such a terrible artist that he inadvertently drew Marten with a look of deep despair and existential dread. He didn't even mean to, but he gave Marten an expression of complete resignation, of final admission that he is dead inside. That's what makes the comic so great, Marten's expression perfectly conveys what any sane straight man would feel when face to face with a full-frontal tranny, and it was ALL AN ACCIDENT because Jacques can't fucking draw.
