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It can be difficult to know where the line is drawn between fidelity and adultery. While definitive adultery is one spouse having sex with someone other than his or her spouse, there is a growing gray area as to just what constitutes "having sex". Bill Clinton quite famously gave us his public view of what he believed was "having sex", though there are quite a lot of folks who clearly do not agree with where he chose to draw the line. But that's just one example of where views vary on this.
It's said that sex happens largely in the brain, and yet adultery seems to require an unequivocal physical contact component. This creates a contradiction that, in our brave new world, only gets more problematic. Husbands keeping a hidden stash of Playboys is one thing, but is someone committing adultery when they conduct sex play solely by e-mail, IM, or an online game? And where does the looming prospect of virtual reality sex come in, outside of an intriguing new area for divorce lawyers to explore/exploit?
Jesus said that to lust after a woman in one's heart is to effectively have committed adultery with her, as far as God's view of the matter is concerned. That seems clear, but the difficulty is that it isn't just about adultery but the Christian view of the inherent sinfulness of our fundamental human nature. Some have even taken this so far as to say that it is a sin for a husband to lust after his own wife, that sexual attraction of any kind is adultery, that anything short of a completely chaste existence is crossing that line, because to give in to anything human is to cheat on God.
That last bit may seen ridiculous, but such views exist and have played their part in the history of both defining and persecuting moral behavior.