Hey SL,
I don't think you are so different. If my husband showed any sign of finding me sexually attractive since he was with OW I might find it more confusing. We had sex twice after BD and both times it was awful - I was sure he was thinking about her (which bothered me because I had only known about her for a month at the time) and the whole thing seemed like a massive effort to him.
My H (I discovered) has a whole host of intimacy issues that I was only semi-aware of in our relationship but only came out to the full extent after BD. Over-dependent on porn, scared of initiating sex (was mostly me who did that), the affair made a lot of sense to me in that sense, because OW was this unknown entity who was a fantasy unfolding in the affair (like a porn star, but without the porn star looks
). Some of our problems may have been exacerbated by our difficulities conceiving D because it is true that it takes a certain spontaneity out of the experience and over a longer period probably can make a man feel that the sex is no longer about him (which was sometimes true because of the necessity of timings etc for conception). Still, I thought that if you loved someone the opportunity to be with them would be pleasing no matter what the circumstances - despite the lack of spontaneity, I always enjoyed it (tmi??). Anyway, with all this baggage I think that this is just another reason his R with OW is doomed, as she becomes more and more "real" and her flaws become evident, and she starts declining sex occasionally, his fantasy will start to fade and he will be faced with real intimacy again, where he has problems.
I also believe H has serious issues surrounding motherhood. Our sex life took a real nose dive after my first was born (even after I had lost the baby weight and was looking pretty good again) but I always attributed it to tiredness with the broken sleep, job stress and all the other concerns that take over in mid-life. It plummeted completely during my pgcy with D four years later and that is also when his MLC started to really take hold. His mother has always been an incredibly strong person in his life (and, though lovely in most ways, controlling and to my mind a bit overbearing as a mother - even now, WHENEVER he has a problem he runs to her and she solves it for him and he is almost 36). So I think that when I became a mother and started to fulfil that role, it was almost inevitable that I could not also be his sexual fantasy anymore. Issues, issues, issues. I actually think that this is one of the main issues that if he does not resolve he may not really emerge from MLC, because he has had such a terribly hard time trying to cut the apron strings.
A lot of the stuff I discovered about my H's attitude to sex was that it was very adolescent and, I think, overly romanticised. He hated the idea that sex could be fun - it was "serious" and romantic at all times.