My instinct was that xh's choice to marry ow was just another bit of running in the hope that some other woman would fix him rather than him take responsibility for his own demons,
I'm with Treasur on this one, this is what resonates with me... and I also see in my H the not wanting to face me, as I have "high standards", and he feels inadequate....
I don't honestly know how they work their way out of that, I stay out of the way now, and have for a long time.
As to whether or not this is necessary? Perhaps for them to find out that it doesn't work, although you'd think mine would have found that out, as I think he holds the record here on the forum for the number of OWs, but I saw a few years ago that he seemed to be becoming himself again, there hadn't been an OW for over a year, it even appeared that he was looking at himself a bit, and then he (quite suddenly, it appeared to me at the time) ran hard -- and yes, it turned out that the latest OW was there. Which is why I agree with Treasur's assessment....
I also don't know much about the OW, but from the little I know she is nothing at all like his mother, nothing at all like me, so goodness knows what. Perhaps it's just that -- something different, maybe just the "someone worse than him" that we often talk about here.