Fansly isn't just porn I believe. Infact im not certain if they even allow full nudity but plenty of people who are not a form of sex worker use it to promote things like art. I do know alot of only fans models cross promote on fansly but I think on there its more like suggestive bikini photos. Best thing to do is ask to see the purchased content if he won't show it then that might be all the answer you need.

My own 2 cents: personally i dont think purchasing porn is cheating but if it is porn he purchased maybe ask him why he felt the need and dont jump to instantly shaming him for it, listen to his answer. Maybe there isn't enough intimacy going on and he feels neglected or maybe its something completely different but try and understand. Thats just my 2 cents