First off: You don't need to deal with the OP of a post being wrong in their reasons for wanting to do something. No site on the entire SE network is the correct place to argue that an OP is wrong for refusing to marry someone due to height, age or caste.
In this case, all you need to deal with is the question, and its quality. It doesn't matter why the OP wants to break up now, and it doesn't matter all that much what they have done so far to get into the situation. What matters is what they have tried so far to get out of it themselves. Right now, all there is is literally a bad, low quality post, that also happens to tick off quite a few of the marks described here. You handle such posts as any other post:
- You ignore the salacious content (reasons OP doesn't want to marry) for now.
- You ask for clarification on the parts that do need to be in the post (about their own behavior, not just that of the girl, what they have tried to tell there will be no marriage, how they handle the daily 'good morning' messages and such) and instruct the OP to edit accordingly.
- You vote accordingly (at least to close, probably also down)
- You wait until the OP edits the post to include the necessary details.
- Then, the salacious content can be edited out so it doesn't become a distraction for the answerers, and you're left with a reasonable question.
Were the question improved to include what OP has been doing to try and cut contact so far, to tell this girl there will be no marriage, when the question is focussed on the behavior of the OP that they have used to try and achieve their goal, only then an answer might be written that could explain that the current interpersonal skills used aren't going to be effective and why, and suggest other ways of behaving during the interactions OP has with this girl that can be backed up by research or personal experience as being effective measures for reaching the goal OP has in mind.
The OP might have an unattainable goal, or they might be 'wrong' in the way they approached things so far. They already know the latter, or they wouldn't be asking here, so no need to 'deal with' that except by perhaps explaining (backed up with experience or research) what about their current approach is exactly working against them. As for questions where the OP has seemingly unattainable goals: either you ask them to clarify the goal, for example, if it seems contradictory.
If the goal is clear however and you don't have an answer as to how to reach the goal, most often the right approach is to leave the question unanswered (by you). Rarely, an answer might explain the difference between 'best friend' and 'acquaintance', and give OP some guidance as to how they might be able to remain 'acquainted' but not 'best friends'. Still, the answer needs to guide OP towards reaching their goal of it being clear there won't be any marriage yet still having as good a relationship as possible under those circumstances. Those answers are notoriously hard to write though, as they are often just opinions ("This goal can never be achieved") and lack the thorough explanation of "You could do this to try and achieve your goal, it may work but also fail, so everything can still all go wrong".
PS: I've also written a pretty lenghty answer to a previous question, about respecting premises vs. challenging them, here. It might be an interesting read for you too, but as I said, it's long, and is mostly about answers instead of questions, while in this case, the question is the most immediate problem. It does touch upon the fact that comments should be asked to clarify questions near the end of it, and if the question were okay enough, then you'd probably be looking at that answer as a guideline for how to answer questions where you disagree with what the OP has as their goal