Don't you think that if prostitution is legal, it can damage society?
Why? People can sell any of their other skills and talents for the benefit of those who want them, for whatever reasons. Why should sex be so very different?
And look at it from the other side: think of the older, perhaps no longer attractive, or shy, or undesirable-for-whatever-reason individual who has lost a spouse, and looks for some of that comfort again I his or her life? Why would you think it so wrong that they should be able to trade something they have (money) to get that something they really desire and truly long for?
Or would you recommend some sort of charity, a SallyAnn for sex workers, to provide that on a charitable basis, like a soup kitchen?
Or do you suppose their loss of a spouse and unattractiveness is God's way of saying, "you've already had enough -- now go be celibate!"?