If this instance was all about math and someone claims that 1+1 = 11, then I can see how being able to downvote them would be a useful feature. But for people posting nudes and porn? I don't really see the point.
To discourage off-topic posts, clickbait titles, spam image hosts, general low quality content, etc. The type of thing which isn't worth trying to report and might not be against specific rules, but still we want less of it.
If large numbers of people don't want to see something in a particular community, perhaps the poster should consider posting it elsewhere or not at all.