Humans, apart from dolphins are the only species to have sex for pleasure. it goes to show that if dolphines d it too then it cant be wrong. It is away of strengthening a bond in a relationship and of getting closer to another human being. If done right its about the closest two people can get. For some people that kind of inimacy is reasuring and in a way necessary. Everybody needs love and it is just a way of showing your love for another person that goes abve and beyond what you do to show love to friends and family. but for others its just about the sex. (the pure physical pleasure)