You can't keep running in and out of my life! Stop IT!

OK....I get it!  You still like your ex-boyfriend or ex-girlfriend and you  might  want to let them  back  into your life...... but there are good reasons why you should not. He or she might be a stand-up person, who, when push comes to shove, makes the right choice.... but they chose to leave, or you told them to leave because it is over. Unfortunately, a friendship with your— ex, someone you  hooked up with for months and even years--doesn't fall under the category of “a good idea, to repeat a personal relationship with” especially  that  he or she’s in another  relationship. He or she did you a favor walking away from you, months or years ago. This ended his/her association with you on a one on one level.
So what happens he or she is temporarily split from their current Love interest? Should you entertain a one night smack down  for old times sake?
To be clear, you and your ex are not lovers anymore. You are two people who used to be in a relationship, you had  dreams, adventures and had sex together; and then you  broke up, for whatever reason. You began a relationship because there were things you liked about each other, and when you both  ended the relationship, you  should not keep each other around to enjoy those earlier traits without the headaches because heartaches will becoming back, just because you decided to be in  an ex-with-ex  relationship.

I actually do believe that some exes (in some cases) can be genuine and platonic friends if they've healed from their relationship and enough time has passed. It’s been years since you and your ex parted ways, but you two aren't “just” friends anymore, will never be that if you have physical relation for old times sake. If you were genuinely friends, you wouldn't have “ended up hooking up, just for "old  times sake” There’s clearly some sexual chemistry at play, and as evidenced, neither of you can resist it. This is not a platonic friendship! This reliving what you both ended for sometime now.
Maybe listening to so many dating and relationship woes over the years has made me some what  cynical, but what sounds like a ploy to spend extra time with him or her when you know he’s or she's vulnerable. That you “ended up hooking up” just makes me believe this even more. In addition, you still “love his or her presence” in your life and you’re taking his or her moving on  years later as a breakup. Again, this is not a platonic friendship, if you get my meaning... this why folks never move one.

Here is an example: He or she had an argument with their current lover. He or She turns to you the EX for some comfort and understanding. You sleep together.. "Oops look at what you made me do".
There may have been a time when your ex was willing to play with fire—that would be with  you—but it seems that he or she is quite serious about keeping his current girlfriend, or her boyfriend, now that he has her back, after a disagreement has been resolved.   He or she  should be, with her or him . not you. Who knows? Maybe the current lover  already sensed the chemistry between the two of you, and distancing themselves from you was a condition that he or she set for taking him or her  back, think about it. Just saying!  Or maybe he, or she let you go as a preemptive measure because things have a tendency to spiral beyond the boundaries with you.

It could also be about appearances in their inner-circle,( that he/she dropped you like your steaming  hot) because it doesn't look as if he was all that serious about wooing his woman back if he was hooking up with his ex. (Technically, he/she  didn't do anything wrong, since he/she  was single for a heart beat.) And when—not if—that story comes out, it looks better if he can say, “I knew it was wrong and I cut off all contact.” His/her ex isn't stupid.... they will know that you are not over..... their lover.

What is clear to me, is that he/she told you that he doesn't want you in his future anymore. It doesn't have so much to do with the girlfriend or boyfriend—he/she had one before and still interacted with you—as with the fact that he has decided he/she doesn't want you to be a part of his life anymore.... should simple and clear. So as much as you think “he/she feels the same way” that you do, his/her actions say otherwise.

His girlfriend (her boyfriend) is his/her priority. And really, this should have happened a long time ago. Despite the length of time that had passed since your relationship ended and he/she dated other women, you still had a pretty big role in his/her life.... because you did not let go... You've may have  just realized that someone outranks you and he/she actually serious about her/him.

 If you were in his girlfriend’s shoes/ her boyfriend's shoes, you would want your man/woman  to make the same choices. You can't keep letting them run in and out of your  life! STOP IT!