Through most of history, most people were poor and did not have a lot of food to eat. There were some taboos in terms what foods you could and could not eat, but one was allowed to eat something each day. There were certain times where fasting was required. But to balance this off, there were festivals, harvest and holidays where feasting was allowed. The result was eating, over time, remained more natural in terms of instinct.
Sex was different. Even if you were poor, and hungry, the opportunity to have sex was there. Sex is pleasurable and could make the hard life more worth while. The problem was, sex could create a wider range of social problems, such as diseases, inner breeding, and lots of children nobody could afford to care for, leading to suffering of children.
If we took away modern birth control, abortion, medicines for STD's, and social welfare for illegitimate children, and funding for related and inherited heath problems, one would see the wide range of problems the past would face, if sex had the same freedom as eating. If AIDS had formed 1000 years ago, it would have been a plague.
The result was rules for behavior were created to regulate sex to minimize all the social problems. No culture could evolve under so much social costs and would be vulnerable to conquest and extinction. The result was repression of the sexual instinct due to law. Repression leads to higher brain potential and more compulsive behavior. This is the nature of prohibition and temptation. The result was more bizarre and unnatural sexual behavior.
For example, although male dogs will hump each other, this is a form of aggression and not about sex. It is a way to define pack order in a way that is not as lethal as fighting and biting. For humans to extrapolate this symbolic aggression into sex, is not natural. Dogs don't take all the way to having sex, but stop at pack order dynamics.
Natural instinct is different from natural plus repression instinct. Modern sex is more of the latter, due to centuries of repression that seems to linger in the brain's firmware.