I've wondered about this as well. Studies are difficult to find. The ostensibly answer is morals/ethics of advertisers which if not already bordering on an oxym

I've wondered about this as well. Studies are difficult to find. The ostensibly answer is morals/ethics of advertisers which if not already bordering on an oxymoron, also makes no sense given that these same advertisers still happily run all the ads they can in e.g. Saudi Arabia. And they're certainly well aware that if 62% of men acknowledge watching porn that that simply means 38% of men lie about watching porn. So it's not even cultural concerns, as might explain e.g. Saudi Arabia. But I think there's probably a very simple answer. People in the mood for porn are going to be substantially focused on the content they're after, rather than getting distracted by your ad. Contrast this against something like a cat pic. Ah that's cute -- then 2 seconds later your mind and attention is wondering, ideally right on over to their ad. The porn ends up being a much worse value proposition than the cat pic, or more generally than any rapidly consumed low-effort low-intensity content. I think the exact same can also explain why advertisers prefer to avoid 'controversial' content. I expect the advertisers ostensibly kowtowing to social media outrage mobs are mostly just using them as useful idiots. They point out controversial and likely highly immersive content. Pulling out not only gets them a few social media white knight points, but also generally results in some degree of controversy which also helps spread their brand name effectively working as free advertising. You might even get things such as people deciding to follow you on social media as a means of virtue signaling. This also points to another ironic point that I think supports the above hypothesis. Advertisers are more than fine with controversy when they themselves are the controversy. See: e.g. Gillette that intentionally planned an advertising campaign exclusively around fanning the flames of a culture war.