I don't know about cheating necessarily, but relationships often become VERY strained after a baby is born. It's a combination of hormones and adapting to new norms that really fukk people up, and cheating is often a side effect of that.
A lot of the time, women devote every waking moment to the baby and the husband feels neglected. But the flip side of that is a woman is required to spend every waking moment caring for a baby and then is expected to "take care" of her husband as well.
Expectations have to change on both sides. If your wife passes out after putting the baby to bed instead of sucking your dikk like she said she would, let her rest. She needs it. If your husband wants his dikk sucked after the baby goes to bed, go ahead and give that jon't a good, loving sucking before you fall asleep. He needs it. Yes, these seem like contradictions, but everyone has to cater to each other.