The way I see this,MOST people,not ME,of course,don't look all that attractive with their clothes on,so seeing these same people naked and having sex in public is an offense to the eye.

After all,it's not like there is a shortage of alleys or abandoned buildings in SF for these people to do this,but on the public sidewalk is a step too far.
Not that it will do any good to arrest them. They don't care about being arrested. It ain't like they are going to lose their jobs.