Also I think as a society we treat sex as different from other activities in a way that isn't really compatible with legal sex work.

For example, if some manager asks the intern to pick up their Dry cleaning as part of their job, some people might frown upon that but I don't think many people think the intern is being wronged or if they are that it isn't really a big deal.

On the other hand, if a manager asks his intern to have sex with him, I think most people agree that this should be illegal and that the manager in question is abusing his power, even if having sex with the manager was listed as a job requirement.

But if your job is only to have sex...suddenly it's ok again.

You can argue that we should treat sex as a normal activity like driving someone around or picking up groceries for them, but currently in society we treat sexual consent differently to prevent these power imbalances and pressured sex in the workplace, and I don't think that treatment is consistent with the sentiment of "sex work is just work"

The power imbalances and pressures are the same for picking up dry cleaning.

Sexual consent is treated differently because of how the laws shaped our views of prostitution. If prostitution, legally speaking, becomes a normal job like serving ice cream or sweeping the floor, then your sideline example naturally follows and is not some kind of exceptional case.