Any act required for the survival of the species must be pleasurable. Ejection of sperm into the proper female location is imperative in order to continue the species. It must be pleasurable for the man or he won't do it. The act of depositing/receiving sperm does not need to be pleasurable for the woman. Rape works to create pregnancy. However, the species requires that infants be fed, so feeding must be pleasurable or females would reject their infants and refuse to nurse them. This set up has to work naturally, without technology as animals don't have bottles. A mother deer doesn't stop to intellectually consider that she must allow the fawn to nurse or it will die. I have seen dogs run around while pups try to nurse so I don't think there is any instinct for nursing that will force a female to lay down and submit to the feeding if she doesn't want to. The goal of nature is to make her want to nurse and the easiest way to do that is to make it pleasurable. Orgasm is just a physical reward, liked to deposit of sperm, or sex, FOR MEN, because only they are required to deposit seeds in the proper place. Physical reward for women happens in response to what women are required to do to keep offspring alive which is to voluntarily feed them instead of leaving them lay hungry or abandoning them. Everything in nature that is mandatory to keep the species alive MUST be pleasurable.