And We'll Call It a Sin

The western world has just come through what is traditionally called the most joyous time of year: a time when (according to the Egyptians) five days were given to us by the gods to enjoy. How did the Egyptians learn of the divine gift? The cresting of the Nile had gone off-schedule: a dead giveaway that the Egyptian calendar was in error. So from those five "extra" days grew such festivals as the Roman Saturnalia and the Christian Christmas. In the Celtic year, winter solstice was already a time of joy because the darkest days of deep winter were over and people could look forward to the lengthening of the day and the return of warmth.
During the revels, it was customary for misrule to be the law. Lords and ladies became servants, and servants played lords and ladies. Everyone had a jolly good time; September birthrates peaked. People did what the self-appointed Christian authorities (the only game in town) labeled "sinning". Christian authorities simply could not allow such jollification, especially when it involved spontaneous acts. It might be possible, church leaders thought, that they themselves would be displaced and have to be the servants--and their servants might be in church --and might even run the church services! (Horror of horrors ... and the ridicule that might be possible--oh, shudder!)
So the individuals who sought power ever more centralized decided that every fun act should be called evil and those committing such acts were sinning. Personally, we don't really know why enjoying oneself is a sin. If everyone is getting a little on the side, then why worry? As the old saying goes, what's good for the goose is good for the gander.
Of course, the authorities followed these ideas of sinning with making monogamous marriage a legal requirement. Now if considering only the preservation of property rights and the protection of children, that was probably a good idea; but extending it to rule out all enjoyment was a step that pagans and Witches vigorously resisted, especially when the least little act or thought could be labeled a sin.
If you didn't attend church every single day of the week, you were sinning, and in the worst case could be judged a heretic--and we all know only too well that being labeled a heretic led to torture and ultimately the fire just because you didn't obey the man-made rules of the elderly, crochety clergy--who probably couldn't get it up any longer anyhow.