More than you might think. Because they don't post regularly doesn't mean they don't browse the forums regularly.
Ultimately, though, what it comes down to is that if there's too much graphic adult language (or images) the site will get slapped with an adult content warning; visitors whose browers are set to block adult content can't access the site; the less people who access the site, the less hits it gets; the less hits it gets, the bigger the chance the admin will decide maintaining it isn't worth the effort or money and call it quits.
Brass tacks: we don't get on folks for the sex/nudity stuff because we're prudes or repressed. It's about making sure the forum remains accessible to as wide an audience as possible.
Besides, this is a horror movie forum. If folks want T&A they can go visit a porn site. It's not like there aren't at least couple thousand out there.