Sexual infidelity (and by this I mean any sexual liaison outside of the one-flesh marriage union) isn’t dangerous primarily because of its temporal consequences. These consequences are quite real, of course, and they are devastating. It is dangerous because it repudiates the gospel, molests the icon of Christ and his church, and forms another, malevolent spiritual union that cannot be disentangled when the situation is left behind.

Russell D. Moore, Tempted and Tried: Temptation and the Triumph of Christ