hi and welcome to beyond blue. Sorry about the delay in responding to you.
It sounds like you were both getting settled and then you find out he has an only fans account which or your sounds like it might have crossed a boundary. Is this a fair summary?
Even though you said he knows you would not like him having an account he did it. This sounds a little like betrayal. While I am not asking how that conversation came up ... have you had a conversation with him about how you feel about this?
And/or find what draws him to the site? (I did a little research and found there is an intimacy and perhaps control thing with only fans - I cold be wrong. If you both watch porn maybe it is a step up in terms of what he was looking for.
Of course, these are all theories and does not necessarily overcome how you are feel about this. Is it a deal breaker? Can you live with him knowing what he does? Is there some sort of compromise moving forward?
If you do see this, I would be interested to know how you got on, or be a sounding board for you.
Listening to you.