From what I understand, I think it was mostly the Protestant revival movements of the 18th and 19th century that are responsible. If you look for the sources that were not curated and censored by those same prudes of the 19th century, the very Catholic people of the Medieval period up until the 18th century were astonishingly earthy and frank about sexual matters.
>I guess you can blame Christianity (maybe Catholicism in particular?) and Victorian propriety for pulling it back the other direction.
Puritan christianity in the US and Europe, after the reformation, yes.
But talk, customs, and such of christian populations throughout the middle ages were just as, if not more, raunchy. Check, e.g. Procopius for some the vibe of what was going on in the Christian empire of Byzantium.
With respect to Catholicism, I was getting at the fig leafs that were applied to classic (Roman and Greek) sculptures and paintings that depicted nudity and the general sentiment behind that movement.
Personally, I've never seen such explicit graffiti. The worst might have been "Call XXX-YYYYYY for a good time" sort of messages.