I think Watson just has historically just focussed on QUANTITY rather than VARIETY, in a "I really enjoy this! I want to do it a LOT!" kind of way as opposed to Holmes' "This is very interesting, I want to see many different variations on a theme for comparative purposes" thing. Their priorities are different.

Apparently I have given Mike & Rudy's sex lives a GREAT DEAL OF THOUGHT. Presumably this is because they are my all time favourite boys.<3