I know many brothels in Buenos Aires but I've just gone to a few of them. The one I like is Caballito Privé. Here's the link: [Solo los usuarios registrados y activos pueden ver los enlaces. ]
It is not in San Telmo but it takes ten minutes (or less) by underground to arrive there. However, I don't know whether the receptionist speaks English.
I'm planning to go there next week. If you want, we can go together. I'm a twenty-three Argentine boy.