For some reason - possibly the use of a "/" symbol, but Male/Female and Female/Female seem to be operating fine - the Male/Male tag doesn't return only Male/Male images. Instead, it seems to be returning all images tagged with "Male", which makes searching for gay shit impossible.

I was going through the alias list and it appears that male/male is an alias for yaoi and yaoi is also an alias for male/male. The search is likely timing out because it is going in circles trying to reach a term that isn't an alias of another.

Search this instead:

male/male -female