Isn't it somewhat the opposite? It's not that "your tax dollars" are supporting that people browsing porn at the library. To prevent people from browsing porn, the library have to spend more of "your tax dollars" to either install filtering software or employ people to walk around and look over people's shoulders.

That's not even considering the far thornier problem that porn filtering programs can often block legitimate web browsing, such as looking for information about sex education, safe sex, STD's etc.