I have never actually heard of a male being coerced into prostitution. I know that some choose it. I've heard of plenty of females being coerced.

> What do Americans call a man who runs a brothel?

A pimp.

> If you want to minimise harm, legalisation strikes me as the obvious move.

Americans in aggregate aren't interested in minimizing harm. In aggregate, our concerns tend to be about morality, and "violence" tends to not get included as inherently immoral. We care more about the relative defenselessness of a victim than that there was a victim in the first place.

That's why we favor charity over systematic aid.