From Forbes (July 25, 2006): "Don't Invest After Sex ... Investors exposed to oxytocin, a hormone commonly released during childbirth, breast feeding, and sexual activity, start acting like dupes in financial matters, says a study just published in Nature. Swiss-led researchers let test subjects play a simple trading game with real money. Those administered a dose of the hormone were twice as likely to overcome any fears of betrayal and display 'maximal trust level.' meaning they were more prone to being hosed by an opponent ..."
I have not read the original article but expect that someone, somwhere, has the evolutionary psychology explanation.