As for prostitution, this is technically consensual sex between 2 adults. It isn't anyone's business what people want to do with their own bodies as long as they aren't forcing it on others. Also we all know that when a man wants something from a woman he has to go buy her things and take her out to dinner or a movie or at least get her a 6 pack and hope to get her drunk (hehe). Women want to see the cash all the time before they have sex with a man, prostitutes are just more honest about it and are usually less expensive.
As for incest, I understand that a history of inbreeding can produce some pretty bad genetic results. Look at the royalty that used to be around and all their problems like "blue blood". I think that in just one case of close blood relatives reproducing, the chances of a genetic abnormality are still pretty small and that it is not until you have a longer history of this inbreeding that real problems become very likely. If no child is made from this, then it doesn't really cause any harm at all. I think many states have already repealed some of their incest laws.
As for homosexual acts. More consenting sex between 2 or more adults. I know here in America and elsewhere around the world there are overpopulation problems. The argument that it is natural for it to be only man and woman so they can make babies is ridiculous. In nature we find animals that have homosexual acts. If we find it in nature in human and in animal, then to say it is unnatural only shows that a person has a poor grasp on his language skills. Also civilizations that have are advanced, at least in their own time, have always had a good or at least a lukewarm feeling towards homosexuality. The Roman Empire is a good example of this. Look at the USA. As we progress here we tolerate it or embrace it even more. A civilization where their medicine and health are advanced, and where their warfare tech and knowledge is advanced, does NOT need babies in a constant reproduction to keep that society functioning. The advanced civilizations needs less of the babies and more of people who do not propagate the species, but instead propagate the culture, to influence and spread that civilizations way of life. So not only is homosexuality natural, but it is desirable for us to embrace it as a sign of our nation doing well.
All in all, I think a few individuals have had incorrect notions pumped in to them for so long that they truly believe what their brains can't make rational arguments for. And these small amount of people feel so badly about their own "deviant" sexual desires that they feel they need to make all of it a crime in order to control themselves. The world will be a better place by far if these people could learn to stay in their own bedrooms, and out of others!