I don't agree with Abby's reply. I don't think that the only way around the situation is to either take a chaperone or get married. If a couple, regardless of age, want to have sex, they are going to FIND a time and place to do it - vacation or no.
I actually think it was WRONG for the parents to GIVE him a cruise, and then say, "You just can't take our daughter with you." That seems like a major set up, or something else is on the parents mind to do that to both him and the daughter. We also do not know why the daughter is still at home, what the parents stand is on other things (i.e. curfews, dates, etc...) and there are several other reasons that might spark their problems with her and going on vacation. I think that there is more to this situation than the daughter let on.
I do agree that she should move out. My brother is 23 and still llives at home, for his own reasons, of course. Although he isn't dating and all of that, there are still rules that he needs to follow while he's there. As for what he does outside of the house, he's an adult. He is allowed to make his own decisions, however, if he came home with 25 drunken friends, I doubt that would go over well.