It's Not Porn, It's an Index
This isn't dirty, we swear.

The titles of these tropes seem like they belong in Nudity Tropes or Sex Tropes, but they aren't (no, Rule 34 doesn't make them so).

Often it's because the title seems that way as a joke (especially Just for Pun titles), and often because the name uses a word that has multiple meanings where one is well known to be about sex. For others, it's a case where the object of affection is given the same amount of drool and adoration as actual porn might be.