How could he dump Marilyn for his ex?

Watched this on TCM yesterday; in other words, I don't have a copy to pause, rewind, rewatch, etc. Missed a minute or two near the end, and was shocked and disappointed to see the picture end with Phil and Jessica back together.
Why? Hell, *I* was in love with Marilyn by then, why wasn't Phil? Why would he go back to that tone-deaf self-absorbed harridan who had left him, and for his boss no less?
Was this some kind of production-code happy-ending to endorse the sanctity of marriage? A man should reconcile with his wife, no matter what, because what God has put together let no man put asunder? Unhappiest of endings, for my money...