It may have been going by the friendliness of the city's people to gays. San Franciso, while very vibrantly gay, isn't exactly known for being a city filled with friendly people, while there's a stereotype in the States called "Minnesota Nice", because apparently we're all too freaking nice here in Minnesota.
You'd use your sister to lure them in and lower their guard against arousal, so they'd be more ready to hit on a guy in public.