Your logic is flawed, and here is why (countering each point in order)
1) If someone's religion believes in polygamy, (and many do) how is it a sin? The 10 commandments were created by man, not by any god.
2)Everyone is different, even clones will develop into different people given the same stimuli.
3) You can get an STD from anyone, in fact 2/3 of the world's population has Herpes. AIDS? people have gotten it from blood transfusions in hospitals!
4) The wife/girlfriend may be frigid and families may be non-existent.
5) A good sex worker will always stroke your ego, she's charging by the hour in most cases, Men with stamina means more money in my pocket.
6)Domestic violence against women exists outside of the sex industry as well. If we give sex workers legal rights then we can report abusive customers and prevent them from abusing someone else, for a time at least.
There will always be a demand for the sex workers, we provide services that someone's partner either cannot or will not do. You may not agree with the choice those of us who have not been forced to be in this line of work, but keep in mind that we do it of our own Free Will.