Reducing people to objects is generally bad.
Only fans also promote sexualization of girls/teens.
It's well established in social research that the patriarchy uses sexual objectification of women to help keep them reduced as second class citizens.
It helps promote the idea that "women want to be sexually assaulted and they like sexual harassment".
Taking blame from the predators.
https://en.wikipedia.org/wiki/Objectification
https://en.wikipedia.org/wiki/Self-objectification
Additionally. Turning intimacy and affection into something to be sold or bought devalues this aspect of human connection.
https://www.thorn.org/research/library/commodified-online-sexual-interactions/
Consent and Commodification: Objections to the Market for Sex Work https://pressbooks.lib.vt.edu/pper/chapter/article-4/
I personally think sex work should be decriminalized for the prostitute but not the johns.
Sex work is not something people do because its a choice but out of need for finances.
Meaning it's a form of coercion and exploitation.
You could argue most work/labor is that. It is. I won't say its not.
But sex work perpetuates harm more (physical, mental and promotes misogyny in our society) and specifically harms women the most. Though it harms anyone working in that industry.