Let me tell you a true story of a guy I know.
He and his wife had a child, and afterwards, she lost interest in sex. Her desire never came back, and that was it. He still loved her and she still loved him, but he wanted to have sex. So he did the 'right thing' and divorced her. Now, their story isn't so bad. He divorced her and it was amicable enough, he still visits almost every day (they live down the street from one another) but they live in different homes.
My first advice to that guy would've been to have a consensual open relationship, but absent that possibility, I think that maybe having 'an affair' would've been a better solution than divorce. The result would've been nearly exactly the same (he doesn't even want a relationship with the women he sleeps with), but they wouldn't have had to live in different locations.
Your view of infidelity and relationships isn't wrong, but it's somewhat incomplete. I can easily come up with a slightly worse case for this--they could've been living in the USA, for instance (they're in the UK). That would've meant that she would've lost any health coverage that he brought to the family through his employment. You can modify this scenario subtly in a lot of ways to make it worse, and sometimes the least bad option is going to be cheating on your partner so you can stay married and in the same house and sane so you can raise your kids properly.
As someone that's consensually non-monogamous, this is all just abstract philosophy to me--I think there's too much emphasis put on sexual fidelity in the first place, and not enough on emotional support and availability. You can be monogamous with someone and still be a wholly shitty partner to them.
So don't be too quick to judge the people that were paying for memberships on the site. Some portion of them are CPOS (cheating pieces of shit, in Savage Love parlance), but some of them are almost certainly people (and, according to the analysis, almost certainly men) that want to stay married but can't live in a sexless marriage anymore, or want to explore other parts of their sexuality that their partner can't provide. You don't know the story.