Maybe because he possesses the unique ability to grasp the truth of the situation, or at the very least, understand his own role in the chain of events. It takes a self-assured, wise person to do that.
Am I missing something, or was he not aware that she had occasionally sexual relationships with other men? That qualifies as an open relationship, meaning she wasn't cheating.
And what good would anger or resentment do him now? For him to be TOO angry about the outcome of what happened, or even regretful, would be to regret having Sarah as a daughter, who he obviously adores.
To me, his detached reaction to Sarah's revelation only served to further show why Diane probably sought passion elsewhere - he doesn't respond to life and its messes with passion and anger. I think there is value and wisdom to his outlook, and I truly respect it.
Believe it or not, there are people who cannot be monogamous, and there are also people who don't mind being married to them, or even prefer it. Sometimes -- not often, but sometimes -- God smiles, and they find each other.
Pretty sure that because of the large number of late in life divorces, and the millions of cheaters, that humanity isn't meant to be monogamous.