A few years ago my best friend from college started dating my ex-boyfriend only a few months after he’d dumped me. It sucked even though the guy and I weren’t in love and had dated for only about six months. They started secretly dating around the time my friend and I rented a beach house with some other friends for the summer. Shit was about to get ugly.

My friend, whom I’ll call Jane, sort of asked my permission after they’d already started sleeping together, as if I wouldn’t figure that out. That meant my ex would be hanging around the beach house at odd hours. I told Jane there was no way I could stomach all of us breakfasting together.

She mostly respected my wishes, but that didn’t make it any easier, particularly because I was single at the time and hating it. I had a meltdown one day after seeing the remnants of a romantic picnic in the backseat of her car (fur blanket, empty bottles of rosé, chipped wineglasses—you might as well just kill me).

Guess what, guys. Girls go absolutely nuts if you dump them and then date their friends. It breaks the golden rule of girl code: No woman shall date her friend’s ex-boyfriend unless the friend is madly, deeply in love with a new guy. Even then it’s dicey. It almost never ends happily ever after for anyone.

Girl code is not about jealousy. It’s about boundaries and respect. When you start sleeping with someone, you submit to some rules of basic human decency. You won’t give her an STD. She won’t lie about birth control. You won’t have to vacation with her parents unless you’re almost engaged. She won’t have to play Grand Theft Auto. You will keep your paws off her friends. So when you meet a group of women, you really have to choose one and stick with your choice or suffer some serious consequences.

My story is hardly unique. Some things just shouldn’t be shared, like toothbrushes, underwear and exes. And I’m not talking about hygiene, though that matters too.

Even if it’s all done aboveboard and everyone pretends it’s no big deal, it’s a big deal. No woman wants to see a friend in a relationship with an ex. It just feels weird. Most close friends tell each other a lot about the person they’re dating and the details of the relationship. Women go way deeper, discussing pretty much everything you could possibly worry about them discussing. Your new girlfriend may pretend her friend, your ex, didn’t tell her all your secrets, but trust me, she definitely did. Now, because of you, they’re probably not speaking to each other. And they’ve probably drawn enemy lines through their mutual friend groups. Girls can be real psychos about this stuff.

Frankly, men are not much better. When you introduce your new girlfriend to your friends, you’re taking a leap of faith. You want them to like her, sure. But you don’t want them to try to steal her. The same rules apply to both genders. You don’t want your best friend from college to sleep with your girlfriend, even after you’ve broken up. And even if your ex-girlfriend is totally over you, she doesn’t want to see her best female friend hanging on your arm.

So, men, if you want to sleep with your buddy’s girlfriend, stop. Go find someone on the internet instead. Don’t hang out with her. Don’t go to her birthday dinner. Unfollow her on Instagram. Be the better man.

If you truly believe your pal’s girlfriend is the girl of your dreams, be patient. Let them drift apart on their own. Act natural and neutral if your friend asks you for advice about the relationship. Only a sociopath would engineer a breakup.

When they do break up—and most couples do break up—continue to keep your distance. I think it’s best to wait a year before you ask your friend if it’s okay to date his ex. You can ask after six months if your friend is happy in a new relationship with a great woman. If you want to date an ex-girlfriend’s female friend, you should at least try to get the ex’s blessing, but it’s really up to the girls to hash it out. Proceed with caution.

I truly didn’t want to date my ex again, but it was the worst breakup of my life. I’m talking about the one with Jane. I really didn’t give a shit about the guy, except I was annoyed he kept trying to have coffee with me to talk about it. Cocktails would definitely have been necessary if I were to agree to such a talk, which I never did. I had enough friends. I didn’t need a new one who had messed up my relationship with an old one.

Of course, Jane and my ex broke up by Christmas. It took years for our friendship to get back on track. We both admittedly acted pretty crazy about the whole thing, but we managed to finally put it behind us.

I can’t help but feel a tad smug that he’s still single and pushing 50. He’s not getting any less bald either. Ooh, it felt good to put that out there. Hell hath no fury and all that.