You Shall Not Commit Adultery
We all know the folks in "Star Trek" enjoy the occasional romantic dalliance. But the "comely alien in every spaceport" way of doing things doesn't fly as well when a potential conquest is married—or appears to be.
Consider "The Man Trap" from the original "Star Trek." Kirk, McCoy, and an expendable henchman beam down to visit Professor Crater and his wife, Nancy—a woman who was once an old flame of the good doctor's. But when McCoy sees Nancy, he's amazed that she hasn't aged a day, and moreover, she's making googly eyes at him.
Turns out, though, there's more going on with Nancy than botox injections. Seems she's actually a salt-hungry alien who can shape shift at will and is flirting with everyone—lusting, as it were, after their salt. Enterprise officials only discover her true identity after several crew members have been seduced to death.
Harsh? Definitely. Still, if there were more shape-shifting, salt-sucking creatures masquerading as married people on earth, infidelity might take a dip downward.