Perhaps this
phenomenon is caused by a guy's knowledge that he no longer must seek a
girlfriend. Most guys have it in their heads that they, for one reason or another, NEED a girlfriend at all times. Because of this, when he meets a
prospective girlfriend, he acts how he assumes he must act in such an encounter. When he already has a girlfriend, he no longer must inhabit this state, and perhaps the lack of this
state of mind is what is attractive...
Pure speculation, mind you.