fucking someone's girlfriend is a string. i've stopped pursuing someone after finding out they're taken a few times, so yes it is true, at least in one case.
String to a trigger perhaps. Usually though, there is hope that their commitment needs are met, and they are only filling their horny needs.
as soon as a girl says that, you speed up everything...the blow job in the parking lot is now on the table her..''i have a boyfriend'' him...''is he here in the bar?'' her...''no'' him..''so lets do another shooter?''