Far from it! In fact it's a veiled invitation to sodomize (in other words fucking the backside) male or female hookers which are for sale there and the actual main source of revenue of the place. In some cases (mainly if the costumer is attractive enough) there is of course the possibility of a free shag as well.
Among gays a backshop is commonly referred as "darkroom" with the light (still) on.
Some pedestrian: "You filthy cum guzzling fag! Hope you'll catch a disease in there and perish!"