Each belief system from Pagans through Christians affects the way we look at our partner and how we think about sex.
While Pagans have always considered it as something natural, Christians condemn it from about the third century. Polytheistic and dualistic views have always encouraged couples to have sex, monotheistic views have somehow changed this view and declared it a sin unless it is done to have children. Yet, they state that sex should be a way of expressing love for each other, even though they consider homosexuality to be a sin, as well. There is no such thing as a good religion to validate one's sexual desires, but if we cannot get ourselves to think individually, then we should not be a part of any religious convention. Perhaps this is why less and less people are religious nowadays, as well. But how have we got here?
According to Hugh Hefner – besides philosophers and historians – it is due to the times when Christianity gained followers all over the world, or rather Europe. Then, the Roman Empire persecuted and executed Christians, who did not share their belief system – that is, polytheism – and obviously, they turned against them. So, as they had a orgies and liberal views on sexuality, they condemned it as thoroughly as they could. One of the evangelists went as far as condemning it even between married partners. Those who did not keep to these views were burnt at stake, or otherwise executed as witches, and women were turned into sinful objects of desire. It still affects our culture, but if we recognize it, we can free ourselves from the guilt it generates in us.