There is also Malachi 2:15.
Has not the one God made you? You belong to him in body and spirit. And what does the one God seek? Godly offspring. So be on your guard, and do not be unfaithful to the wife of your youth.
Ok, so you probably wanted to post Malachi, not Proverbs.
It seems translations differ in this word, so I looked up the Hebrew:
acted deceitfully (1), acted treacherously (2), betrayed (1), betrays (1), deal (1), deal treacherously (10), deal very treacherously (1), deals treacherously (1), dealt treacherously (8), dealt very treacherously (1), faithless (2), treacherous (15), treacherous one still deals treacherously (1), treacherous deal (1), treacherously (1), unfairness (1).
From the context I think its about "faithfulness" in her old years, to take care about her even when she does not look good anymore. If you think it is about not taking another wife, can you show why do you think so?