Lust is an unsavory word, and it is certainly an unsavory topic for me to address, but there is good reason why in some traditions it is known as the most deadly of the seven deadly sins (see, for example, Henry Fairlie’s excellent The Seven Deadly Sins Today ).
Why is lust such a deadly sin? Well, in addition to the completely Spirit-destroying impact it has upon our souls, I think it is a sin because it defiles the highest and holiest relationship God gives us in mortality—the love that a man and a woman have for each other and the desire that couple has to bring children into a family intended to be forever. Someone said once that true love must include the idea of permanence. True love endures. But lust changes as quickly as it can turn a pornographic page or glance at yet another potential object for gratification walking by, male or female. True love we are absolutely giddy about—as I am about Sister Holland; we shout it from the housetops. But lust is characterized by shame and stealth and is almost pathologically clandestine—the later and darker the hour the better, with a double-bolted door just in case. Love makes us instinctively reach out to God and other people. Lust, on the other hand, is anything but godly and celebrates self-indulgence. Love comes with open hands and open heart; lust comes with only an open appetite.
These are just some of the reasons that prostituting the true meaning of love—either with imagination or another person—is so destructive. It destroys that which is second only to our faith in God—namely, faith in those we love. It shakes the pillars of trust upon which present—or future—love is built, and it takes a long time to rebuild that trust when it is lost. (Ensign, May 2010)
However costumed or made up, lust is no substitute for love; actually, brothers and sisters, it chokes out the development of real love, causing “the love of many [to] wax cold” (Matt. 24:12). No wonder we are told to “bridle all [our] passions, that [we] may be filled with love” (Alma 38:12). Otherwise, oozing passions fill the available soul space, and double occupancy is not possible. (“The Seventh Commandment: A Shield,” Ensign, November 2001, p.78)