when you can't physically mate with the one person you consider your soulmate because they/you are hooked up with someone else, but your souls can mate just being together, or thinking about oneanother.
dean & donna
dean : I cant believe that we cant be together, I'm going nuts thinking about you all the time.
donna : Lets just let our souls mate for now!
dean : you mean soul mating?
donna : yea, our souls can mate any time they want to.
dean: thats ridiculous!