Because sexual liberation - an ideal of sex outwith monogamous marriage with children - has been the wedge that has incrementally destroyed most of the Christian West; and because we live in the most pervasively and expansively sexualized society in the history of the world.
How could Christianity not be obsessed with that which has all-but eliminated it from public life?
We now recognize that there can be no compromise on this issue (of course there must be mercy to repentant individuals): this is a battle which Christianity has to win.
What kind of arguments should Christians use in discussing sexual ethics?
NOT utitlitarian arguments - not arguments about how Christian ethics make people happier or relieve suffering or make for a more humane or more efficient society. This is to use the One Ring to fight Sauron.
NOT rational arguments. It won't work. Modern people can only think in one-, or two-step, sound bites.
DO USE natural law arguments - about what everybody knows and feels, spontaneously, instinctively.
DO USE arguments based on tradition - the overwhelming consensus of our best and most Christian ancestors. Do point out that modern notions are just a few decades old, and question the arrogance of modern man (us being who we are) - could we really have superceded and can we really reject all the greats and Saints of the past?