I have never been able to grasp what it is about so many men that makes them think that purchased sex can be in any way an enjoyable or fulfilling experience. I finally came to the conclusion that it must be in some way tied in with the perceived 'unmanliness' of masturbation - the thinking that makes 'wanker' an insult. Childish.
I was brought up in a port city, like you, and am aware that men trapped in an all-male environment seem to find it necessary to seek 'relief' as soon as they get out of said environment. Again, this seems a little immature to me. I blame it on the social constructs which prevent men finding relief together in mutually-pleasurable, consensual activity.
I would also suggest that the frequent abuse of prositutes is in part due to the fact that the sexual act has in fact proved to be unsatisfying., engendering the rage that coarse and stupid men are overly prone to.
As to the truly perverse, those whose 'needs' are so fantastical or inherently unpleasant or dangerous as to require the purchase of partners or victims, they are as beyond understanding as they are beneath contempt. I find that contempt, along with much mockery, to be a key feature of the works of de Sade.
I will add only one more observation. I have found that there are some women who regard marital sex in a disturbingly cold, transactional light: "I saw a nice coat/frock/bracelet in Expeensive Shop today. I'll let the old man have his way on Friday night, then when we come in town on Saturday, he'll buy it for me!" These, mind you are, women I know (including my own mother). Women whose husbands I also know and who are men who keep their marriage vows. It seems that the practice of transactional sex is not confined to prostution.
Then again, as a heteroromantic asexual who has never felt 'randy' in his life, I may be missing something!