Something which can happen is lust. The meng has supposedly found the woman of his dreams.. and yet she (alone) isn't good enough for him; so he covets other women, preferably in secret - for the renewed thrill of seduction, for voyeuring misadventures. Is it jealousy that lingers in his heart, driving him off the marriage rails? Boredom?
Or is it that his wife - confiding in him about a fantasy she's had with a naval officer - it is revealed that a woman's heart, even under marriage, still remains full of secrets, which sends our hero (and a Real Human Being!) off on his own infidelity quest around New York City, until he finds himself satisfied by topping his wife's fantasies somehow
Or that on either side of the marriage, the husband/wife come to the revelation that their innate flaws would threaten to overthrow what happiness was promised. All work and no play made Jack a dull boy, and Clarice was a nervous wreck of Woody Allen
It may depend on their instinctive drives priority: the need for practicality, sociality and union - and if it doesn't get satisfied. Of course, if you're looking to incite suspicion in an already healthy marriage, you'll need at least one of the partners to react not-so-goodly upon hints of cheating.
edited 7th Apr '12 11:52:01 PM by QQQQQ