"Don't love people, baby! You must restrict your romantic interest to just me! Polyamory is strange and freakish! Your culture is wrong!"
Hers is no MORE valid than his either.
Dick was raised to be monogamous, and it's certainly not wrong to keep to that. He also holds that the vows of marriage should be respected, even if it was a marriage of convenience, and sleeping with someone else's wife is not a good thing to do.