For it to work for me, there has to be an element of chemistry. As I've gotten older, I think my radar for the mechanical, going through the motions vibe has gotten more sensitive.
The punts where, even though the girl was "hot" and "sexy", but I couldn't get into it, were because of the lack of genuine chemistry. I had a long anticipated punt with the fabled Priya91 a few months ago, and while I found her objectively very sexy - and told her so - I didn't get off on it as much as I wanted to as I felt she was (very expertly) going through the motions.