Well, at least you wouldn’t -knowingly- cheat or stray. Lacey did kiss Keith (ew, almost to the same degree as the Cora-Rumple kiss, but he was kind of a victim there). And MM and David under the curse had their own share of cheatingness (all hugely complicated by the fact they were cursed, of course–who WAS David actually cheating on, Kathryn or MM?).
But I think the reason that Cora felt she had to kill Daniel was that a person wouldn’t willingly leave her true love for another. The times when Snowing did, they were doing it to save the other’s life, compelled by outside forces, just as when Rumple pushes Belle away it’s usually to protect her. And for all Hook’s suggestive comments, I get the feeling that during the time he was with Milah, he didn’t even -see- other women that way.
Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing