Indeed there is a broader conversation about what amounts to "inappropriate conduct" outside the workplace. Assuming the person isn't doing anything illegal and maintains their work at arm's length, does the employer get to police what they do outside of their working hours?

If a teacher hustling as a sex worker in her free time is grounds for dismissal, would a teacher hiring the services of a sex worker in his free time also be a fireable offense?

Yes and no.

Part of the issue for both is that posting stupid shit non-anonymously on social media does call your judgment into question, especially if you're in a public-facing role and your behaviour be liability to your employer. Say Nazi stuff on Facebook and your employer fires you? Well, that's a life lesson about consequences of your own behaviour.

...and that's also the case here, though OnlyFans allows a modicum of plausible deniability because it requires the viewer to actively and decisively source the content in question. The problem, for the content creator, is that the internet is leaky, and what happens on OnlyFans doesn't stay on OnlyFans.

This is separate from the compensation issue. For sure EAs are paid incredibly poorly, and while there's liability issues, there are larger moral ones about slut-shaming someone trying to make rent through legal means.