There’s an old saying in San Francisco: Market Street is straight, until it reaches the Castro.

Cartographically true, it refers to the city’s famously gay mecca. But these days, there are concerns that the neighborhood is becoming slightly less bent.

A Different Light Bookstore, which specialized in gay and lesbian literature for 26 years, shut last weekend — the latest in a series of closings of longtime businesses that were gay-owned and operated or catered to gay clientele. The neighborhood is littered with empty storefronts.

Amid this gloom, however, there is one burst of excitement that has crowds lining up, drawing visitors from throughout the Bay Area and beyond: Sunday brunch at Lime on Market.

With “bottomless Mimosas,” the restaurant and club has become so popular that it can take weeks to get a reservation. Patrons regularly defy stanchions and block sidewalks as they wait to cram inside where techno pop music blares at rock concert decibels.