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.
Contrast Index of Exact Trope Titles.
Pages in category "It's Not Porn, It's an Index"
The following 20 pages are in this category, out of 20 total.
Media in category "It's Not Porn, It's an Index"
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