When you go out on the town to hit the bars, the point is generally to mix with the other gender. Sometimes, however, it would preferable not to have loopy girls or obnoxious guys hanging around.
In the Gender Filtered Bar a person can optionally avoid the other gender. The main bar entrance leads
to a common room where guys and girls can mix freely. Apart from the main room are two gender-specific sub-bars for guys-only and girls-only, respectively. These are entered via the guys restroom and girls restroom, effectively filtering out the other gender. A guy goes into his restroom from the main barroom, and proceeds through a back door into the guys-only bar which is equipped with guy stuff. Same for the women.
The Gender Filtered Bar is especially useful for women. When a creepy guy latches on, she simply goes to powder her nose ... and goes on through to the girls-only bar to peacefully sip on a vodka gimlet while watching the Brit version of What Not to Wear on tv.
I have been to a lot of bars and haven't seen anything quite like this, although there are bars in New Orleans with special restroom entrances. Back doors in both men and women's restrooms lead to another bar (strip bar) where both genders can go for a different theme.