That’s basically the essence of western sexual morality. Anyone can bang your wife but you (marital rape is illegal), and any kid deserve your money, except yours. Okay that’s a joke. But if you interpret lack of reward as punishment, and anything punished as prohibited, and consider marriage as a form of prostitution, it won’t be too far of to accept.
It’s not that bad considering the alternatives though. It all comes from what I call, inevitability of power.
So, basically different people favor different ideologies. Different cultures are controlled by different people and hence tend to have different values. Interests of the people that are in control decide the prevailing morality in a society.