What you said was "I don't know any normal, decent, secure, well-balanced person who has had an affair."
Even assuming there is an agreed definition of "decent, secure, normal" and all the rest, you simply don't know this. You do not know the deepest intimacies of everyone you know, even the ones you really like. I realise you could say that anyone who has an affair isn't decent etc, but that's the "no true Scotsman" bit.
And I actually think a lot of people who have affairs are pretty secure. That's often what gives them the confidence to do it. I'm amazed when I think of some of the ways I've known some men to be caught. They were so careless, I have to assume they were so secure that they just thought the universe would hide it for them because it would do whatever they wanted. Don't we have a thread every other day about someone seeing their friend's husband, photos and all, on a dating website?
Affairs aren't all the same. Some cheaters are selfish tail chasers, some are fundamentally good but weak and lack strength of character, some are in mental crisis, some have been treated like dirt by their partners - cheating really isn't the only way to shit on someone. Anything that starts with an assumption that all cheaters are a Certain Type of Person - usually one you didn't much like to start with - is based on a flawed premise.