If all brands allowed their ads to appear next to adult content, then it wouldn't be any special association for any particular brand, just another outlet.
So I guess it has more to do with the historical prudery of some countries, when an ad appearing next to adult content would trigger angry letters to the editor, editorials, and so on from "concerned citizens".
That said, advertisers didn't seem to have much issue advertising all kinds of stuff on Playboy back in the day, or FHM and the like today...
I guess itβs because Playboy and FHM are somehow considered tasteful and for connoisseurs?
Well yeah. Brands aren't going to put in the effort to solve a thorny collective action problem just to open up a bit more ad space. They're trying to make money, not repair broken social norms.
Where an ad is seen can be just as (if not more) important to the advertiser as the ad itself. So, if your site serves up adult content -- you can guarantee that companies with large ad budgets won't be buying ad space.