I don't think a person should be fired for having an OnlyFans account.
I just wonder where the line shifts to. Employers could come up with any excuse to fire you.
It's one thing if there is a clear conflict of interest. Like, you couldn't be the CEO of McDonald's and Burger King.
That aside though, it shouldn't matter what else you do to make additional income, provided you're not breaking any laws.
In theory you could work at Chik Fil A and if you have a part time job at Draft Kings and your Chil Fil A manager decides they don't like that because it goes against their values against gambling, they could fire you. And that would be ridiculous in my opinion.
The long term impacts of this stuff can have unforseen repercussions.