No, it is not appropriate at all. Such stories involve private and sexual content that should be kept within the boundaries of a private and consensual relationship. Sharing this kind of story in public violates privacy and is not in line with good social ethics.
No. It is not proper English. The use of 'cum' in this context is vulgar, and 'ovr' is an incorrect spelling. A proper phrase would be something like 'calling him to come over for a story about sex' if that was the intended meaning, but even that would be a rather racy or inappropriate statement in most contexts.
This is a rather unclear and ungrammatical phrase. 'Cum' is a vulgar term for orgasm, and 'ovr' might be a misspelling of 'over'. But overall, it's not a proper or commonly understood expression in normal language.
Well, 'cum' can be a vulgar term. If we look at it in a more general sense, it might seem like a very personal and inappropriate story about observing something related to one's boyfriend in an improper context. But this kind of expression contains vulgar content.