maybe what you would qualify as 'good' and what he really wants to see are worlds apart? and if this is true, perhaps he doesn't want to upset you by admitting he's into that. also, he may not want you thinking poorly of him because of his 'preferences'.
you seem like about 100 000 000 miles away from being a prude (from what I've read around here, you seem like a great girl actually), but everyone has their own little dark spots they like to keep for themselves.
could that be it, you think?