One situation where I think it's perfectly rational to use sexual services is for mentally handicapped people who have no realistic options for actual relationships. I live in Denmark where prostitution is legal on some conditions. The healthcare staff sometimes have to order prostitutes for their clients to cope with their urges and thereby avoiding violent situations from someone getting too frustrated. The client pays themselves and it's both men and women using the option. The sex workers in these jobs are usually not found in back alleys or dodgy websites but through personal networks. It's still very taboo.

I wonder how these clients are treated in countries where it is outright illegal. Probably not at all, or by illegal methods.

The main problem seems to be trafficking, not the sex services. Everyone does something for money that they really don't want to do, like going to the office 40 hours every week.

If it was possible and required to verify the consensuality, it would probably remove a lot of the illegal services, and more legal services could thrive. There'll always be ways to work around it, so it's a difficult thing to address.