From a quick google search surveys show that American's almost unanimously find cheating morally wrong, and 60% of all children in America are born to married parents.

>Outside Christian communities, sex, family, and marriage are not inherently related

I don't think you can say that at all. Looking at the marriage rates in looks like the majority of sex is probably happening in married relationships, even outside of conservative Christian communities. The average American may be OK with sex outside of marriage, but I think they're still pretty related in most people's minds.