not all skills are taught in school. the cheap ones are only selling a warm body to have sex with. the good ones, who charge hundreds or thousands of dollars, certainly are skilled at sex. they are the type to know sex instruction books inside and out, performing every trick in the book if the customer pays for it. giving a proper blowjob is difficult and requires plenty of skill (excepting on adolescent boys). there are tricks involving the tongue, you have to know how to handle teeth, how much pressure from your lips, reading response and adjusting accordingly, how to take him in the mouth and when to do it, you must know the degrees of sensitivity of the different areas and which movements with the tongue and mouthparts to perform there, you have to take into account speed. building up to the point of orgasm in the most eloquent way possible is an art. it requires a background of knowledge and practice, no less a skill than cutting hair or massaging people.
not all people who go to prostitutes are married. if they are, they probably shouldn't be. if they must stay unhappily married for whatever reason, i doubt the wife would care much so long as she gets to go out and do the same thing.
there are some who do not believe in monogamy and a prostitute is obviously one such person. why should she care if he is having sex with or in relationships with other women or men?