And as a result her plan is to utterly humiliate Caitlin by dragging her home and exposing her completely innocent sexual relationship with Archie. If she just wanted to tell Declan about Maud, she didn't need to drag a teenager into it, and create controversy over her burgeoning sexuality. She didn't need Caitlin to be any part of that, so even without the storm, her behaviour towards Caitlin was nasty and cruel.
Maybe I missed stuff, but it wasn’t clear to me that any of that was part of her plan? It wasn’t really clear to me that she had a plan, she was deeply emotional and obviously felt she couldn’t bear to be in the house with Tony a minute longer. I agree that driving off in that storm with Caitlin was deeply irresponsible and reckless but it wasn’t at all clear to me she was planning to humiliate Caitlin, I think you could just as well say she was trying to extract Caitlin from the storm that was going on in the Baddingham family, which honestly to me seemed reasonable enough (had there not been the real-life storm). I think most people would probably rather not have a comparative stranger of a teenager in the house while their marriage was breaking down.
And lastly, Tony is a shit and a creep and he and Monica married in presumably the early sixties, where her options were extremely limited. But actually, marrying someone who you will never, ever be attracted to, and denying them physical, romantic and sexual attraction is pretty horrible. She's not Tony's victim, they were two people using each other and what she did to him was every bit as bad as what he did to her. And her whole 'I didn't used to be' about her lack of sexual desire wasn't a deserved insult, she wasn't going to be attracted to any man, aiming that at him when she knows her lack of attraction to him hurt him, was cruel. It's not like Tony doesn't deserve cruelty aimed at him, but that particular cruelty wasn't satisfying because that was on her, not him.
I don’t think this take reflects the reality of gay people in the 60s, or later. Let’s not forget that gay ‘conversion’ therapy is still not banned in the UK even in 2026. When I was growing up in the early 1990s with same-sex feelings and trying to find information about them (which was not easy to do, not least because discussion of it was banned in schools under Section 28), the entry on ‘homosexuality’ that I found in my parents’ Encyclopaedia Britannica still said that it was possible for most gay people’s sexuality to be changed through therapy. There’s no reason to think that Monica might not have believed, or hoped, when she married Tony that her sexuality could be changed by the marriage. That’s what the society around her would have been telling her. Yes it’s awful for him as well as her that he was married to someone who couldn’t be attracted to him, but I don’t think we can assume she had the freedom of choice or even the knowledge in making that decision that you’re claiming she did.