I don't think it matters whether the exchange of value involves currency or not.

There are things that most people find to be unethical to "sell" (exchange for value in a transaction). Those include actual human beings (slavery), military or political influence (bribery), and murder.

I believe that sex work in often included in that list because of a lengthy deep history of protection of "bloodlines." Of course, there has always been sex work, but those who offered such services - especially women - were by definition unconcerned about their own "bloodline," which must then mean that their "bloodline" was not worth protecting. That meant that providers of sex work were necessarily "lower" people.

Today, and especially in the global West, the notion of "bloodlines" is more associated with bigotry than high status. That's why we're calling it "sex work" now instead of "prostitution," for example - and sex work is more socially acceptable now, even if it's not super high on that scale. Because that cultural thing about "bloodlines" is well-entrenched and runs very deep.