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I mean, branding is a thing too. You don't see J&J slapping their name on porn sites either, even though that's a cheap form of advertising with massive impression numbers.
I recently added some Taiwanese porn sites to my wank rotation. And while some of it is good, many studios have this annoying habit of putting temporary tattoos of adverts on the performers' bodies, usually on an asscheek and on the lower abdomen. I can't read the ads, but I'm pretty sure they are for gambling websites.
Bruce Bethke, the guy who actually invented cyberpunk and wrote the story Cyberpunk, wrote a book Head Crash. In which the VR hotsuit includes a "ProctoProd®" for bass. Bruce's predictions have turned out more accurate than anyone else's.
Heck, they would shove them in your dreams if they can (and they will, using neuralink lol); and then find crevices on the laws to navigate through, like crabs side-walking to move forward.
But other people can. It's all about the target audience. You would actually get paid, to show ads in your ass, when someone has a clear view into it. People with ads up their asses are just the medium to deliver the ads.
They already shove ads down our throats, so it's not a big stretch of imagination to say that yes, they would totally stick ads up our arse is they could.