I do not agree with your "every human interaction is transactional" analysis and frankly I think it is a disgusting side effect of capitalism's influence on your mind.

But, I mean... they'd have a job, right? The whole jobs guarantee/universal employment thing is pretty key. Unless we're talking about some far future utopia where scarcity is completely over and the robots do everything, people still have to work. "From each according to his abilities." Or even "he who does not work does not eat." I really don't have much hope in a project that makes work completely optional and just assumes everything will get done out of some shared sense of duty.

The debate is then over whether or not prostitution would be officially recognized as legitimate employment in a socialist society. If it's some anarch-ish society where you can choose your employment with near-complete freedom but then have to justify it to society to get your stipends or something, then maaayyybeee, but if it's some sort of planned economy then absolutely not--seriously, what the fuck. (Currently imagining someone writing a grant application to strip at parties.)

If it's your side gig you do to afford more hobby supplies or something then knock yourself out, I guess, but an economy where everyone needs a side gig is edging pretty close to "failed state" territory (sorry, Cuba! And America!).

should it be illegal for someone to do that?

Of course prostitution should not be illegal. Rather, it should be made impossible and inconceivable. To get to the heart of my point, sex with a person you're paying to have sex with you cannot be consensual. This shouldn't be a controversial statement. In no ideal society should rape be not only commonly accepted, but systematic and institutionalized. That's all.

In other words, insofar as there exist "illegal" acts, prostitution should be seen as a "legal" but unfortunate side effect of some failure in the way the society is structured.