merchants don't deal with mastercard, they deal with an acquiring bank
of which there are hundreds
no doubt one of which will be happy to take the business
and I very doubt it's the case, the card networks simply don't care, given you can buy adult entertainment from millions of websites
the acquirer will care if it pushes up their chargeback rate, but this is normally solved by the merchant by paying a couple of bps more
it's a negotiating tactic, nothing more
merchants don't deal with mastercard, they deal with an acquiring bank
of which there are hundreds
no doubt one of which will be happy to take the business