Harry Denton's Starlight Room is great! Sort of romantic and cheesy at once. Another great place for singles is the Beauty Bar on 19th and Mission. Good DJs, fun dance floor and the bartenders have a famously heavy pour.
I used to go to the Beauty Bar all the time and I always preferred weeknights because it's a very small place so it gets pretty crowded on the weekends. And then...I got married. Bye bye Beauty Bar.