One of several definitions referring to a woman who is sexually "open" to try anything her partner wants in the bedroom or other places including public locations, i.e. elevators, bathroom, kitchen, etc.
It has been suggested that that the ultimate fantasy womangirlfriend acts like "Queen" in social circles but when it comes to pleasing her man becomes a sexually, no holds barred, "Mugrosa!"
I would refer to the movie, "Fatal Attraction" to get a partial idea of a "Mugrosa" is capable of!
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi