A friend once told me, “You should never break up with someone without a back-up plan.” I took her advice to heart. It just made sense. You wouldn’t leave an apartment without finding a new place to live first, so why would you leave a relationship without a solid plan of where to get your orgasms and feelings going forward? Still, there are times when one unexpectedly finds oneself in a period of sexual vagrancy—maybe you got dumped, or a bad fight ended your relationship abruptly, or your back-up plan just fell through. It happens to the best of us. It’s during this delicate and lonely state that we find ourselves doing what one should never do: sleeping with the ex.

You know the drill. You break up with a guy—you know he’s not the person you want to spend the rest of your life with—but you’re alone, you’re horny, and he’s available enough, so every now and again, you succumb. Before you know it, six months or a year has gone by and you’re still tied up in this sort-of-together-but-not-really thing with your ex. You’re not seriously seeing anyone, because the strings with the old guy are preventing you from being emotionally open enough to start something with a new guy. It’s not friends with benefits, exactly: It’s the post-relationship crutch.

Case in point. After my ex-boyfriend and I broke up, we didn’t speak for four months. It felt clear that we’d both moved on, and I had started seeing someone else. Then he had to come over to my apartment to pick up some plants he’d left behind. No big deal, I wasn’t stressing about it. I didn’t even redo my eyeliner before he showed up. But somehow this plant exchange turned into a casual cup of tea; into me being bent over the kitchen table; into me crying on the floor about all the great times we had together. (This emotional purge came as he ever so slowly backed out of the apartment, potted aloe in hand, mind you.) Suddenly, it seemed all the time I’d spent moving on from the relationship had been in vain. All the feelings came flooding back—so violently, in fact, that I felt physically nauseous afterward. I felt like the recovering addict who convinces himself that he can have just one drink, and, the next thing he knows, has a needle in his arm. I had to start my sobriety all over again, from the beginning.

My friend Max, a 35-year-old musician, has been sleeping with his ex-girlfriend for over two years now. (I’ve changed his name and a few details to protect his privacy.) Which basically means, in my eyes anyway, that they’re still dating, though both of them are adamant that they’re not together. Neither of them has dated anyone seriously since the breakup, and it’s pretty clear that their continued involvement is serving as a roadblock to their meeting other people. Max insists he isn’t sleeping with his ex only because it’s easy, but because it’s just genuinely rare that you meet someone you have a real connection with.

“The problem is that everyone else pales in comparison to her,” Max told me. “The relationship stopped working so long ago—it was over even before we officially ended it—but I’d be lying to myself if I said there wasn’t still something there, or that we weren’t still sexually attracted to each other.” He went on: “There’s nothing rational about it. We get along terribly. We’re bad for each other. But then there’s just this thing when we’re together that’s so charged and so hot, and that doesn’t die, no matter how unhealthy the relationship is. When I try not to see her, and then I finally give in, those feelings come back tenfold.”

Max also said that when he and his ex try to get back together for real, as they have many times, it just doesn’t work. Despite loving each other, he said, they are very different people. “Another problem,” he added, “is that that my ex thinks I really messed up because I started sleeping with another girl before we officially ended it. It’s still very much an open wound.” In other words, when we start hooking up with an ex after a breakup, we don’t just get to magically start from scratch. The baggage from the relationship is still there, and the reasons you broke up in the first place are probably still valid.

Of course, all of this residual drama can make the sex more exciting. “Since the breakup,” he said, “my ex-girlfriend has definitely tried to hurt me as payback for how bad I made her feel. It’s a constant power struggle, but that can make things more intense, especially the sex, which is hotter than ever. It’s similar to when people have affairs. It’s rarely about the other person, but rather about wanting excitement and attention and the scandal of it all.” Essentially, sometimes it feels good to do something bad.

In my own experience, sleeping with an ex has been more about possession than excitement. There have been multiple times when I knew I didn’t want to be dating a guy anymore, but the thought of him being with someone else was so hurtful, I couldn’t let him go. At a certain point, these relationships just became cock blocks. After a breakup, sex can be used as a kind of manipulation—you keep sleeping with someone you’ve fallen out of love with just to keep them from being fully free. It can also be a great way of reminding an ex of all the things they no longer have. Think of the Mad Men episode when Betty seduced Don at their kids’ summer camp, well after they both had remarried. It was a moment of such power for Betty, she irreverently sexual in her jean shorts, Don weak at the knees. It seemed Betty seduced Don not for her own pleasure, but simply to prove that she could. And as morally questionable as that may be, it worked.

Letting go of a partner is a multistep process. First, we have to relinquish the physical relationship and deal with the fact that our ex is sleeping with other people, which of course can be upsetting. But it’s when your ex starts seriously seeing someone new that you begin to dwell on all the more intimate moments. Realizing that he is now having those moments with someone else induces a whole other level of jealousy and sadness. It’s surrendering the emotional closeness, not the sex, which really hurts. “The scariest thing,” Max told me, “is thinking that someone else loves my ex girlfriend just as much as I did—that they have the thing that we had, which at one time felt so sacred and untouchable.” But as scary and painful as it is, it needs to be done, otherwise you’re just holding yourself back.