The original sin is one of the peculiar things Christians believe. How could they believe babies are sinful?
Augustine wrote in the 300’s. There were competing writers of the time who thought he was wrong. The sin of sex was made BIG by Augustine and may never have made it into the category of big sins had it not been for him. And, it was his own personal obsession with sex that made him generalize about all sin.
When he was a teenager, he was in a bath with his father when he had a spontaneous erection. He seemed to have pondered this experience for the rest of his life. He had sex with several women and fathered a child while not married.
My take is that sex was so pleasurable to him he concluded it must be sinful. And, since his bath experience happened with no conscious effort on his part it must be that sin is in humans from birth. The first thing we have is sin.
Another explanation for the pleasure of sex is that humans need to reproduce. Those who found pleasure from sex reproduced, those who did not died out. It had nothing to do with religion.
But, there is no fame and fortune to be gained with such a simple explanation. Augustine became famous because he named a sin and judged others. Giving power of some over others is good for one’s career. It’s especially helpful if the ones you give power to are wealthy.