How Sin Becomes Outdated

The more modern definition of sin really should be any action or habit that is a detriment to your life and others. 

In ancient times, the infant mortality rate was high and life expectancy was a fraction of what it is now.  So given proper historical context, it's almost reasonable that any sexual behavior other than that used strictly for breeding would be deemed as "sinful."  Ancient societies needed to keep growing to maintain competition with neighboring foreign countries and families needed a high child count for both labor and simply because not all children were even expected to survive unto adulthood.  So sex really was "sacred" back then, and for it to not be used strictly for that purpose would have been seen as endangering to the growing, fragile community.

Fast forward to now, we're at something like +8 billion with near triple the life expectancy.  We dominate the planet on a magnitude unfathomable to those tribes whose definition of the known world would have been what is now the Middle East.  So the idea of any relationship between consensual adults being a "sin" is no longer practically relevant, only a stubborn holdover of a cultural intolerance that has no place in modern society.