From a PURE Christian perspective (as opposed to Christian religion today) then the issue is
that consensual sex is only between married partners. I can find reasons why casual sex causes
personal and societal harm, as most Christian churches do, but that's not the point of pure
Christianty. And finding these personal and societal reasons is actually 'self righteousness' to
the bible, despite the good intentions.
Until recent adultery attracted the death penalty in Western societies, as it still does in Islamic
and many pagan socities, so we can be clear about where the bible stood.
In the Gospels there was a specific moral law - don't be an adulterer, don't hate your brother,
don't steal, don't cheat, hate your enemy etc.. And this was a prerequisite for personal savation.
So morals were tied to salvation.
Today's secular morality is classic self righteousness, and as my stats below show, over time
it mutates. All the wars of history had 'moral' overtones to them.