Ross supposedly being able to leave Demelza for Elizabeth
In Season 2, after Ross "ravishes" Elizabeth
everyone seems to assume he can simply cast Demelza aside and be with Elizabeth. But how would this even be possible, when Demelza is his lawfully wedded wife and this is a time when divorce is notoriously difficult even for people from the uppermost echelons of the society, and probably impossible to achieve for a penniless country squire like Ross? (Yes, he obviously can simply abandon his wife and start living with Elizabeth out of wedlock, but that doesn't seem like it would be a viable option for Elizabeth, or something which upper class ladies like Aunt Agatha or Caroline Penvenen could so casually suggest.)
- You also have to wonder why anyone would think that was a good idea in the first place. Elizabeth needs money as well as a husband, and Ross isn't exactly rolling in it. The only way Ross could have divorced Demelza was if she was infertile, which she obviously wasn't, so it really makes no sense.
- Snobbery has something to do with it. Since Demelza was merely a scullery maid when Ross married her, and Aunt Agatha still refers to her as one, they think he can set her aside and act like it was never a lawful marriage and/or have it annulled. It would help that the marriage witnesses were Jud and Prudie, a couple of lowlives.
- In the book, Demelza lies about her age on the marriage certificate. She says she's 18 when she's a few weeks shy. That might be enough to annul the marriage. Of course, it would still ruin the reputations of everyone involved.