Noting here in case it surprises anyone, I've just reworked the way the search page behaves, in response to user reports about unexpected behaviour around the gender codes.
When you selected /f or /m, it would include or exclude stories that had submissives of that gender. I assumed, like the users having issues, that the F and M options would work the same way, but for dominants. Instead they were acting to search for stories with a dominant or submissive of that gender. So it was possible to look for stories with a submissive of a particular gender, but not a dominant of a particular gender. That could lead to some unexpected results, for example, if you selected green /m and red M, you were essentially saying "only stories that have males, but also don't have any males", and it would give you back zero results.
I've changed the F and M options to search for dominants of that gender instead, which I think will be more in line with what people expected. If anyone was making use of that any-role gender search, I can add it back in, maybe with some clear labeling about what the difference is, but for now I think the simpler set of options is more useful.