I think saying "No Black" or "no men under 30" or any sort of pre-selection is discriminatory, based on the values and rules of our society. I'm not saying it's not right to have criterias, afterall the SPs are sharing with their clients what's most intimate, but it is discriminatory.
Now, imagine the day that paying for sex becomes legal here, the reverse will occur. All laws, rules and values we have set on us will apply to this industry, just like we apply it to others. This discrimination (or whatever how we want to call it) will then become illegal, just like we cannot restrict a 19 years old boy to go to a strip club.
I once went in Germany where it is fully legal, and there was no restrictions whatsoever on who could have the service other than having the minimum age for it.