Probably because the admins are allowing us to respect the rules (which we're abusing), rather than setting up the forum to allow only female users. We probably should be banned from the girls section, or it will more than likely keep happening. TBH, I do think you have a good point.
How about making every user put male/female in their profiles, and allowing access to the girls area automatically based on the profile setting? Just a suggestion...
Personally I just click "new posts" at the top, and if a thread title comes up that I'm interested in, I'll go to it, whatever section it's in. I dare say that this is what most people are doing.