I'm more bothered by the broad swaths of popular Christian belief that are based on either relatively creative translation or bizarre misinterpretations. For instance, how anyone can get "don't masturbate" out of the story of Onan is beyond me; that's like saying that the moral of Goldilocks and the Three Bears is to not eat porridge. Add to this any proscription on premarital sex; premarital sex within a long-term committed relationship between adults isn't a question anybody back then even really had to consider, since marriage was much younger and courtship worked differently, and any prohibition based on the word "fornication" can be argued to be about something fairly different in the first place.
I can explain
exactly how this came about. Early in the Christian era, the Church became dominated by an Last Times sect that believed that the End of Days was just around the corner. The leader of this group, before his conversion, was an extreme hedonist (from what I've read, he made Hefner and Flynt look like prudes). Saddled with massive guilt over his previous excesses, and believing that humanity needed to purify itself so as to be ready when Christ showed up (the concept of the Rapture wouldn't be around for more than a millenium and a half at this point, in case you're confused), so he issued edicts against virtually everything pleasurable, especially non-procreative sex. (This is the root of ALL Christian homophobia. Prior to this edict, gay relationships were as accepted as any other, based on the records I've read. Because gay sex had no reproductive function, it came under heavier fire than the heterosexual variety.) After a few centuries, as the Last Days sect loast influence, most of the edicts were overruled, but the sexual ones remained.