The restriction in our AUP is indeed because payment processors (unfortunately) still frown upon platforms which help facilitate the sale of 'adult content'. Without it, it's unlikely we'll be accepted to e.g. Stripe Connect and/or PayPal for Marketplaces further down the line.
first thing I did was go to that page and ctr-f > "sex" - then "porn" - nothing was found.. so it created interest..
then I saw "are prohibited by the payment processing service; or " near the top and went oh my - many people are not going to get that this is the cop out -
I came back to the comment thread here and did the ctrl-f 'adult' - and found the answer.