Genuinely didn't even know that site was still online.
Is this also self sabotaging a relationship? Sure, but if they wanted out of a relationship then they could do that via divorce. Actively searching for a new sexual partner seems to me to be what it appears to be on the tin... looking for someone new to sleep with.
People often don't know if they really want to end a relationship or not and even if they did, breaking up is hard to do. They don't just get divorced one day, there's usually years of lead up. And as people start to grow apart or to resent each other, they can act out in all kinds of ways, including cheating. And if someone is too afraid to pull the trigger and actually end a relationship, getting caught cheating is a great shortcut.
As a married dad, a few years I suspected that a married mother, at the bus stop, was feeling me out for an affair. I was both flattered and terrified.
A year later she and her husband initiated divorce.
The thing is, if my wife and I were different kinds of people, jumping into an affair like that would be totally fine. We aren't, but I don't judge people who do. (If you're religious, think of concepts like Free Will and Non-Judgement.)
(It's one of the reasons why I never went back to Burning Man, I was really creeped out when an old, married couple propositioned me and a girl I was hitting on for an orgy.)