I think you should take at least a week to think about your values. Would you be ok continuing to date and possibly marry this lady when you know she'd continue with OnlyFans? (A week of thinking is needed to ensure you aren't making an off the cuff judgment)
You could have a conversation with her as well (after your week of thinking), letting her know that it's something that, while she might not feel its' cheating, you are extremely uncomfortable with. If your relationship will work, she has to be understanding of your point of view, but you also have to understand this might be her livelihood. You would have to prepare to support her financially (to the quality of life she is used to) if you request she give it up.
Personally I am a woman and I would absolutely not be comfortable with my spouse (man or woman) having an OnlyFans. That said, if I really loved the girl I'd try to work things through first. If you off the cuff dump her and don't even try to communicate, you probably never really loved her