But even besides that, you said it yourself, that you believe they are less likely to get caught so why would they get caught leading to the divorce? It may increase infidelity but it could easily make more marriages happier as well. Not everyone is the same. Some feel that any sexual contact is infidelity, others feel only certain sexual acts are infidelity, and still others feel it is only infidelity if there is an emotional connection. If a spouse is fine with their spouse going to a prostitute, then it is none of your business. It is probably better if they are having problems that revolve around sex, which some couples do, that if sex with someone else is considered an option, then it is available through a prostitute (regulated) rather than just someone picked up for a one-night stand or worse, someone the person knows, which turns into a major drama.
I just don't think that you can really say that it would increase divorce.