I'm wondering, if while in NRE, he felt like you only spent time with him out of obligation. Runic Wolf would say that to me occasionally in the early days of our quad, when I didn't have a handle on the NRE (after being poly for years, this was the first relationship I'd been in where there was a fiery burning passion for someone other than him and I hadn't heard of the dangers of NRE at that point.) You say that the nights increased from 2 to 4-5, was that while you were still in NRE or after Mr. got his girlfriend? I can see it being a combination of both, especially if you were in anyway wrapped up in NRE.
From his posts it seems like he took your entire relationship with your OSO(s) to mean that you wanted less of a connection/ commitment to him. So he gave you what he thought you wanted, got a girlfriend AND then you snapped out of NRE. Perhaps because you started getting to know your girlfriend better and became concerned about her relationship with your boyfriend? So now you are reminded that you need/ want to focus on your marriage, but he feels (IMHO) that it's too little too late or even insincere?