I'm not sure about the timing, but sometime after J was born, K got to be friends with a woman who lived in the area. I don't even remember what prompted him to seek out a friendship, but he did - oh wait, this was when he was staying home with J after I had to go back to work (J was only six weeks old - some maternity leave, huh?). They got to be really good friends, and spent a lot of time together. He met a couple of other people around this same time, and when J was about four months old and K had been acting mighty strange for a few days, he finally broke down and told me he'd been unfaithful. Not with the woman he got to be really good friends with, but with one of the other people he'd met. He was so upset and so freaked out by the thought that he might lose me and J over it that he was scaring me, and I was really more worried for him than I was concerned for myself or furious over what had happened. I *was* really pissed that it happened in my house, though (but then, where else would it have happened, as K was home taking care of J during that time). He swore it was a one-time deal, that it meant nothing to him, and so on, and I chose to give him another chance. Had the infidelity been our only issue, the separation might not ever have happened.
But then there was the woman he got to be really good friends with - I'll call her JG. They'd go out driving on a Saturday, and be gone for hours - eight, ten, twelve hours at a time. This left me home with J, who was getting bigger and busier by this point - working full time and carrying the weight of child care on the weekends didn't do a whole lot to improve my mood, and I started trying to tell K how I felt. I'd tell him he was taking me for granted, he'd agree, and nothing would change. If she called and wanted to go do something, he'd cut and run, no matter what it might leave me in the middle of. I cried plenty over this, because it hurt like hell. But it didn't seem to matter how I told or asked or pleaded, it seemed like whatever I said fell on deaf ears. Eventually I got to the point where I decided I could either continue to hurt over this or I could say, screw it, because it became clear that K's actions weren't going to change. I opted to say screw it, and the pain gradually became less. I think, although I might not have been consciously aware of it at the time, that I was growing less and less happy with my marriage all this time - I've since told K that this period was what did the most damage to our relationship.