Pretty sure it's more a matter of location. Normal people only masturbate in the privacy of their own homes.
I know San Francisco is a very progressive city, but I don't think they're yet ready to embrace homeless people jerking it on the sidewalk outside the library.
That being said, your comment comes across as reductive to the homeless population as being nothing more than a snide, tongue-in-cheek joke.
This "Normal people" messaging is what allows us to abstract homeless people into this "other" group, which is dangerous and inhumane. Like it or not, these people with all of their problems are a part of us all.
I mean, I get that you want to see homeless people better supported. But championing their right to masturbate in public is I think going to get your cause more opponents than supporters.
It is only a joke until you actually see that, then it is no longer a joke, just sad reality.