I like the idea of seeing hot, sexy, come fuck me women in the bars at hotels.  Are there any good hang-outs in Union Square for such women?

You'll definintely enjoy the The Redwood Room off the lobby of the Clift Hotel, about 2 short blocks from Union Square.  It will likely meet all your expectations.

http://www.clifthotel.com/en-us/#/explore/?id=/clift-san-francisco-redwood-room/

Yes, there are always ladies at the Clift Hotel who are ready and waiting. It is preferable that you have a room in a nearby hotel. I heard that the Clift checks room keys by the elevator.