I think she's probably right, I know a couple of very "happily married" cheaters. They are cheating for some of the reasons that EP talks about, to be a new version of themselves, to be the white knight, to be a porn star hero.. Whatever. I don't disagree with her thesis, but what she's missing is that cheating is like asymmetric warfare. Let me explain (and bring by my theory of happiness points to do so).
Let's just take a stance and say that cheating makes the cheater happy. I think as much as we don't want it to be true, it is, if it didn't make them happy, they wouldn't do it, right? But that happiness comes at a cost, at the cost of the BS's happiness. And it's not a 1-1 trade. And that's the fundamental problem, the world is a less happy place after an A ends. Let's take a typical couple in a typical marriage, and let's say that the husband is getting ready to cheat. Right now, the soon to be WH is pretty happy, let's call it a 6 (1-10 scale). BW is happier, let's call it a 7. The other couple, the WW is not all that happy, 3 happiness points, where the other husband, he's happy, 8 happiness points.
The A kicks off, the WH and WW are having a blast. The WH gets 3 happiness points, the WW gets 2 happiness points. If you look at this in isolation (which I believe EP is doing) it looks pretty good, overall it's +5 happiness points right? Now, eventually that math won't work, both APs will become more unhappy in their marriage, as will the BS's, but, let's just look at the "honeymoon" phase of a new A. Everyone is cruising along, WH is getting more sex, WW is getting all the romance she's missing at home. Nirvana, or close, right?
Well, then DD comes. Now, where do all these totals land? Well, on the 1-10 scale, both the BS go to a 0. Or a strongly negative number, but, since I made the scale, let's just call it a 0, the worst pain you've ever felt. The WH chucks his AP under the bus, and she's devastated, the WH and her BH are both treating her like trash. She goes to a 1 happiness point, nearly the worst pain she's ever felt. The WH is scrambling to save his marriage, has to deal with a devastated wife, probably isn't getting any sex anymore, and is living in constant turmoil. Let's say he lands at 3 happiness points.
So, in this hypothetical example, we lost almost ALL the happiness points across these 4 people, the world is a shittier place, you have 4 people who are miserable for a variety of reasons and will remain miserable for a long, long time (much longer than your typical A lasts). Adding +2 happiness points takes ALL the happiness points from the BS's. It's asymmetric, the WS get's a bit happier, but the BS sees their world fall to pieces. And that's the problem, I have no doubt that A's add happiness for some people, but the amount of happiness they add is NOTHING compared to the amount of heartache they cause after discovery. It's like someone letting you borrow 100 bucks today at 10,000% daily interest. You're a little happier when you get the 100 bucks (day 1), but when the interest accrues and you see the amount owed on day 30 you realize that you mortgaged your happiness to buy a new set of headphones. The happiness added by the 100 bucks is nothing compared to the pain of realizing you're now going to lose your house because of a really deal on a loan. Incidentally, a deal that this one sided is illegal, you're not allowed to make a loan at 10,000% daily interest. But that's what an A is, it's a tiny bump in happiness compared to the bill that will come due at the end of the month.
A's make the world a shittier place for all of us, the more they happen, the more unhappy/broken people we have. Even if it leads to D, the M/W the betrayed dates next (and perhaps for the rest of their lives) gets to pay the bill for the affair (often). It hurts people MUCH more than it lifts up the AP's.
I've heard a few people say "The sex better have been mindblowing/fantastic, because at least then they got something out of it". I don't feel that way, but I understand the sentiment. So much pain and loss for what? A few "I love you's" an orgasm and a romp in a the backseat of a car or bent over a dumpster? That sex better have been life altering, mind bending, best sex ever because that A has forever changed my life for the worse. And in nearly every (every, in fact, I've never seen anyone argue otherwise) case it's not. It's just some "I love you's" and a few orgasms. The price paid for that is insane.