They screen out those that sleep with prostitutes, sleep with those from some countries, travel to some places(for a period of time), tattoos(for a year), etc.
The reason is simple. Health.
EUPHENISM ALERT..EUPHENISM ALERT!!!!!
let's call it like it is....they put their male sex organs in the other guys anal cavity and they do it exclusively that way....in the process they wreck the fragile blood vessels that are in the anal cavity and thus, they are prone to becoming infected with all sorts of diseases....