You don't grasp the bigger picture here. Western society is built on Christianity. Undermine the foundation of Christianity, and you undermine Western civilization.
Like it or not, this nation floats in a sea of Christian principles. We are currently operating only on the inertia of what the society has been before. As these Christian principles erode, so too will other moral prohibitions.
We will eventually get to wholesale slaughter because people will realize that there is nothing sacred about life.
My point stated more eloquently:
Read more: http://www.americanthinker.com/articles/2017/04/the_war_against_reality.html#ixzz4e4leuYVy
Your understanding of history, and the role of religion in the West is severely lacking. Western society is built on Platonic philosophy/principles and by extension, Enlightenment thought. Much of what you think are "Christian principles" either pre-date Christianity or have developed alongside it.
Christianity has certainly played a role in the development of Western Society, that is an obvious truism, but Christianity itself has evolved and changed over the millenia....so its a moving target. Christianity was not some monolithic entity imposing its imprint on the Western world, rather, it was a melange of widely varied philosophies developed from a core concept centered around the teachings of Jesus. Teachings which in their original form bear striking similarities, for example, to those of Buddha. Since the very early church, concepts of what those teachings mean...and which teaching are valid...have varied dramatically.
Further, life is sacred to many varying faiths...moreso, in some cases (Buddhism for example), than is the case in Christianity...so your concern about wholesale slaughter related to the absence of Christianity is ridiculous. That sort of thing happens with and without Christianity, as has been the case throughout history. There ARE faiths, such as Islam, that encourage barbarism and brutality...but there are many others that have ethics and history far more pacifist and respecting of life.
The question of gay marriage has nothing do with religious belief...if a particular religion disapproves, it need not sanction gay marriage within its own practice. Some faiths, including many Christian groups, do sanction marriage with THEIR practice. The problem is that government should have no role in the marriage business...it should neither recognize, nor offer benefit or detraction, to anyone based on marital status. Marriage is a private religious matter, not a governmental one.