Okay, the facts. This is a man who has been living the life of a single man for months while you were struggling with a new baby and PND. He made you feel that you were being unreasonable for objecting to this and that your suspicions were based on your illness, so that you wondered yourself whether you were suffering from severe mental illness and paranoia.
When you posted your original thread, people who mercifully aren't suffering from PND validated your suspicions and told you that his behaviour was a dealbreaker on its own, regardless of infidelity. You confronted again and got an admission that he had slept with someone once, but that it was now over.
When that seemed like a dealbreaker, he retracted his story and told you that he had only told you this to shut you up. You posted again, people told you he was lying again and advised you to check his phone bills. You did and found the same number over and over again and phoned the OW, who confirmed that this was an active and current relationship.
He's therefore got absolutely zero credit rating for the truth, until faced with incontrovertible evidence.
You'll need more than a week of abject apologies and sorrow (which is about being caught rather than for what he's done) to believe anything he says, I'd imagine?
It doesn't surprise me in the least that you were having regular sex (once a week with a new baby and PND, is certainly regular, btw) and that he is claiming inattention and loneliness. But in answer to your question, no all men don't feel like this.
People don't have affairs when they aren't getting their needs met in a relationship. They have affairs when they aren't meeting the needs of their partner, when they've stopped giving to their relationship. So you need to reverse your thinking and recall what your own diary might have said.
I bet you weren't feeling cherished or adored and I bet that you weren't having your needs met by him either, am I right? However, as a grown-up, I expect you reasoned that an adult's needs are secondary when an infant's needs are all-consuming? If you felt like this, it didn't occur to you to start living the life of a single woman and use a man for sex while your H was left at home, depressed and holding the baby, did it?
Stop thinking that this is about his needs not being met, because that is flawed thinking. Think instead about the needs he wasn't meeting in you.