WW here. Not justifying the WH's choice to have the affair in any manner- as many have stated, his healthy choices were to work on the marriage, or leave. He chose to abuse his wife with an affair instead of support her through a very difficult time with a newborn.
This post really spoke to me, as it mirrors a lot that took place in my situation and what I've found had to be worked on. I'm not there yet, but beginning to get some better understanding and perspective, and that's what I'm hoping to offer here.
I'm hoping to add some background and perspective from a spouse that was denied physical and emotional affection and support for over 1.5years while in the midst of post partum depression.
To be clear, I was lashing out in anger at my husband instead of coming to him with humility, which completely killed his desire for intimacy with me. That coupled with the Catholic NFP we were doing and the fact that I got pregnant with my daughter 3mos "too early" made it impossible for him to trust that we wouldn't get accidentally pregnant again. (FYI- antibiotics can give you a secondary ovulation in a cycle, not just produce signs- read that AFTER in a med journal study
). This all lead to him rejecting me physically- no hugs, no hand holding, no cuddling, not even sleeping together most nights let alone making out or kissing or spooning or ANYTHING. A year and a half. Add to that an unwanted (on my part) vasectomy before our sex life started again.
I cannot begin to tell you the level of DEVASTATION and REJECTION and PAIN I lived with those years and YEARS after when I would give him sex knowing he found me to be an unfit mother and unworthy of affection and praise in his eyes. I felt like a WHORE for him- granting him sex so he wouldn't leave me. I was DESPERATE for his affection, DESPERATE for his approval and REJECTED on so many levels by his withholding. Sex was the only physical contact with me initiated by him during that time. No affection, no words of affirmation and no real emotional support. Not even support in my counseling for my depression (there was a $$ crunch and he asked if my sessions were truly necessary at that point, I was desperate for his approval and stopped them).
At the same time as all this, he was DEVASTATED by my yelling, HURT by arguing, FRUSTRATED by my denial of my own contributions to our problems in our marriage. The only time he was pleased with me at that point was after sex, or after a large purchase for him or a week away from me and the kids and $$ spent hunting or on a gun or hunting equipment.
So, there's a lot of dynamics that could be going on here. OP could be dealing with PPD and general overwhelming exhaustion from being a 1st time mom. WH could be dealing with a deep sense of rejection in her stonewalling him for physical sex and affection.
However, if you're going to move on from this, there's some work both of you could do to understand each other and then care for each other.
That said, sexual intimacy is a deep seated need in men and part of their natural wiring and instincts. They communicate their intimacy and emotions (when they have a healthy sexuality) through physical interaction with their spouse. It sounds like your anger from his unrealistic expectations post partum and your acting out (in the form of stonewalling) made your husband feel deeply rejected on an emotional and spiritual level. That he was vulnerable to an affair in reaction to this hurt is on him to heal and figure out. His decision to act out in that abusive manner is likewise on him and he needs to heal from that. That is something I am currently working on myself. However, unless you understand his feelings of rejection and hurt, you're not going to be able to develop the empathy you need to find peace and healing moving forward for yourself. He needs to learn to make himself vulnerable to you and discuss his feelings of rejection and pain in a constructive manner so he can heal and change enough for you both to move on from this.
Women on the other hand, need emotional intimacy and voluntary non-sexual physical affection in order to feel safe enough to physically express themselves with their husband. Emotional intimacy and affection communicate to us that we are special to our mate and not just another piece of meat to satisfy their physical desires. Our sex drives are (typically) wired to be receptive rather than initiative in that department. In other words, female sexual desire can be induced by a husband that is meeting her emotional needs for security and voluntary affection (non-sexual touching). It sounds like your husband was doing neither for you during this time. It also sounds like you feel reduced to the level of a tool for his masturbation instead of a cherished participant in the act. That's on him to own and on him to understand so he can help you heal. However, understanding this about yourself and communicating that constructively and in a vulnerable manner to your husband is what's needed going forward if you're going to feel heard and heal.
I wish you both the best moving forward and hope for both your sakes that you can gain understanding and empathy for each other- whether you're able to repair your marriage or not.