I once counseled a married couple who came in to therapy in the middle of their divorce process. They didn't have any children but wanted to remain friends, and they wanted me to facilitate an amicable divorce, which was hard given how completely they seemed to hate each other. These two, I'll call them Chad and Ellen, had the fresh bitterness of a couple that is still trying to hold onto a failing relationship, rather than a couple who has already gone through the tough work of divorce. I was often befuddled at how short their fuses seemed to be with each other. One fight in my office became so intense that I asked them to physically separate to cool off, and after a moment, I went to check on them individually. That is when both Chad and Ellen informed me that they were still sleeping together.
Suddenly everything about them made sense. Of course they were still infuriated with each other--they'd cleanly separated everything except for their genitals!
If sex complicates just a friendship (watch any recent TV show or movie if you want to see examples), can you imagine how much it can complicate something as already complicated as a divorce?
There are a million reasons why having sex with your ex-spouse regularly is a terrible idea. At the minimum, it's a Seinfeldian case of double-dipping. So why do people do it?
The biggest reason seems to be that ex-sex is comfortable. Post-divorce, the world can feel harsh and full of jagged edges. The person you used to turn to for comfort may not belong to you anymore, but if you can feel their comfort again for a few minutes here and there, it can be hard to resist. But staying coitally connected to an ex will only damage you in the long run. You have so much work to do in figuring out how to renegotiate the world as a single person, and having orgasms with an ex doesn't foster any of that work.
Regardless of what emotions you consciously put onto your encounters, the fact remains that you are keeping yourself holed up in the past instead of working towards a healthier future. You may be getting your needs conveniently taken care of, but you're also closing yourself off to meeting anyone new. You deserve more than just convenience sex, you deserve meaningful, hot sex with someone you could have a future with, rather than someone you have a past with. Worse, you may delude yourself into thinking that somehow, the two of you could get back together, but as the old adage goes, an ex is an ex for a reason.
Another justification I've heard from divorcees is that sex with a spouse is so much hotter once they're a former spouse. This makes a certain amount of sense, given that suddenly, sex with this person has gone from being a everyday option to being illicit, but don't fall into this trap either. If it takes this much drama and pain to make your sex life hot, keep looking.
Some people advocate for the "one last time" sexual encounter with an ex-spouse, and if it's just one time, this may be the only situation where I could accept ex-sex as an option. Perhaps you want some closure, or perhaps you want to force some sort of catharsis on a singular event. But it's tricky to force sex into being cathartic (what positions do you use?!), and it may be too hard to keep your emotions from getting away from you.
In my professional life, I have never seen a couple that is able to keep their sex life neatly compartmentalized and active post-divorce. It's just too messy. But in my personal life, I have a friend who has been regularly sleeping with her ex-husband for about six months. It's sex with someone she's familiar with, and frankly, she's scared of putting herself back out there to the general population. She insists that she's able to grow, heal, and learn while still getting laid by someone with whom things can't possibly get any messier than they already have been.
I am dubious, but I also see that there a million ways to heal after a divorce. It's just hard to imagine that continuing part of your married life well after your marriage has died is one of them.