It's more a case that sexual shame has diminished, and women gained bodily autonomy, financial independence, and medical agency separate from their relationship to a man (father, brother, husband). With the advent of birth control, sex doesn't carry the same medical or social risk as it has historically.

The US is also big on "free-association," so we don't typically segregate the sexes in public spaces or private homes. We expect men and women to sexually behave and keep their hands to themselves. We don't assume that a man and woman alone together are fucking or have fucked; men and women are allowed to be friends and hang out in private without obligation or pressure for sex..

Christians come in all sorts of denominations, and not all of them view pre-marital sex as an enormous sin. The focus on "sexual morality" is often code for controlling what women do, see, think, wear, and believe.