You are wrong about this. I guess what you are saying is that it is impossible for men and women to be platonic friends, business partners, or co workers, and that all male female relationships are about sex.
Have you ever gone on a date with someone you knew that you were not going to marry. Most people have. Perhaps you want to go to dinner or a show and dont want to do it alone. Perhaps you want to have someone to talk with at dinner or about the show youve just seen and see everyone else is coupled and dont want to be alone. Perhaps you have professional or intellectual interests you like to share.
When a man and women go out on a date it most likely is the man who pays for the date, because 1) it is the custom and expected of any decent man and 2) most men make more money than most women.
When a man and woman go on a cruise it is simply a long date.
There are also circumstances where a man may have business either on the cruise or at the ports and may be going to business events where he is expected to have a woman with him (if he does business at the port he may be able to take a tax deduction as a business expense).
Also at least one of these posts offered her companionship to women as well as men. There may well be some elderly or disabled man or woman who has difficulty getting around and needs a companion or pehaps a single parent or couple who desires an au pair.
I do not know what prostitutes charge but its my guess that if a man wants to pay for sex he can do so for a whole lot less money than the cost of a cruise.
Your quote was "It's the nature of man" perhaps it is but these posts are being made by women not men, promiscuity is not the nature of women, so if everyone is up front with what they expect and what is or is not included there should be no problem.