The hard fact is that having sex with your ex-boyfriend, ex-husband, ex-partner, ex-one-night-stand, ex-whatever is playing with fire on too many levels for it ever to be the truly right thing to do. Strong feelings will resurface. You may long for him in ways that will surprise you. You may have to fight an overpowering lust. Powerful sex-triggered hormones in your body can make you think you're falling in love again. You will have amnesia about the reasons you're no longer together and be sucked into a fantasy of "happily ever after" this time. You will be tempted to think only of the good times, and you may even find yourself thinking about what could have been, what might still be ... stop that.
Have you ever had sex with an ex?