The prevailing explanation for this is that women trust the decisions made by other women. A woman may subconsciously say to herself, "If he's good enough for her, then he's good enough for me." If she then acts on this adulterous impulse, she is officially a "mate poacher."
Men, it seems, are well aware of how women think on this matter. If women are drawn to men who are in relationships, then maybe men will choose to flaunt their partners, particularly if they are attractive. Indeed, new research shows exactly that: Men (and women) like to flaunt their sexy partners to their peers.
In a recent study, psychologists assigned random photographs of men and women to college-aged participants, and asked the participants to imagine the person in the photograph as their romantic partner. Subjects were then given a choice of two locations where they could meet their supposed partner. One meeting place was full of undergraduates, and the other was full of administrators.
Where the participants chose to meet with their partner depended on how attractive that partner was. If the partner was attractive, the participants chose to meet where the undergraduates were, "flaunting" their sexy partners. If the participant thought that their partner was unattractive, they chose to hang out with the administrators. (The control group was given a similar story, but wasn't shown photographs of partners.)
So, why did the subjects who received attractive partners "flaunt" them? Judging from survey responses, the researchers concluded that participants who had attractive partners believed that their social status and desirability would be improved among their peers. Perhaps flaunting an attractive partner serves as a "signal" that you're a hot commodity, advertising just how wealthy, superior and perhaps even how genetically gifted you are.
And, since relationships often aren't permanent, it's good to maintain your status and sexual desirability among your peers -- just in case you find yourself single and ready to mingle. (Perhaps this explains why celebrities insist on dragging around arm candy everywhere they go?)
This study, like many psychology studies, is limited by its use of American undergraduates, who likely don't have characteristics that are generalizable to all of humanity. Also, it might have been more informative to use a study design in which a participant gets to flaunt his attractive partner in front of members of the same or opposite sex. For example, if a man believes his female partner is attractive, would he choose to flaunt her in front of other men or in front of women? If the former, then flaunting is probably about increasing social status; if the latter, then flaunting is probably about increasing desirability.
The study was published in PLoS ONE.
Source: Winegard BM, Winegard B, Geary DC (2013). "If You've Got It, Flaunt It: Humans Flaunt Attractive Partners to Enhance Their Status and Desirability." PLoS ONE 8(8): e72000. doi:10.1371/journal.pone.0072000