Sexual acts, in and of themselves, are not illegal (obviously - people would become very scarce if that were true). Giving people money is not illegal (obviously - charities would be out of business if that were the case). If a woman has sex with a total stranger, then sure, I would class that as an immoral act with very dire consequences, but it isn't illegal, because she has free will under the law to have sex with whomever she wants to (assuming everyone involved is a consenting adult of course). And if a sexual partner gives her money, that is very suspicious, but again not illegal. I'm married. I have a sexual relationship with my wife and I give her money all the time. How can you, legally speaking, separate that from what a prostitute does?
All I'm trying to make clear is that solicitation (by whoever) is what makes prostitution illegal. In other words, if a prostitute or a third party (a pimp) asks for money for sexual favors, or offers her body on the understanding she will receive payment. A sexual encounter, no matter how casual, may be very unwise for all the reasons you list, but it is not illegal. It is the selling of sex that is illegal.
If you can't see the difference, I doubt anything I'm going to say can make much difference, but here goes anyway....
There is no sense in bringing marriage into it as an example. Marriage is a commitment between a man and woman to be husband and wife. It's his obligation to support her, sex notwithstanding, based on the marriage contract and his obligation as a husband. The fact that the coincidentally happen to have sex some times and he gives her money sometimes, is irrelevant because he is not buying sexual favors from her in a business transaction as prostitution does.
The very definition of prostitution involves selling the sex, not just having it.
Any female who just gives herself away is not a prostitute. A tramp, yes, but not a prostitute.
Prostitution, selling sex for money or favors, is NOT a victimless crime. Often the prostitutes are controlled to the point of slavery, by pimps. They can and do spread disease which can be passed onto others by either partner.
Just because there are not immediate and dire consequences to prostitution does not mean it's a victimless crime.