I have the exact problem from a practical sense than you you have, DfMIL, but oddly enough, same kind of consequences. My H's parents are barely involved in his life at all! You would think that would be a good thing, but because of that, H is so screwed in the head and emotionally immature, he doesn't know how to deal with conflict either. He doesn't know how to heal a relationship rather than running from it.
Like I've posted before, I decided the "whether there's an A or not an A" issue was not a big a deal as the bottom line issue of H's escapism behavior, and lack of commitment. Like you, I'm one of these truly committed people. Can't see myself dating, even if separated, just like you stated a couple of pages back. It would be wrong to me. On the other hand, H has the slightest feeling of "not being in love anymore" and he's looking for the quickest escape route! In your case, it's your In-laws. In my case, it was looking for old gf's and XBox for sure...and possibly an EA.
They're both running and need to face their issues.
