It wouldn’t really be “sex work” if they weren’t doing it in exchange for something would it?
Yes, we have currency as a placeholder for trading goods directly but people who perform sex acts for other goods like drugs are just as stigmatized and no currency was involved.
And if people are just having sex with a fun of it then it’s not sex work either, it’s just sex, which is less stigmatized now then it was 30 years ago but it still has a stigma attached to it, otherwise slurs like “skank” and “slut” wouldn’t exist.