I think Yukino's weird behavior is because there might be some truth to the supposed affair by her former husband. In her mind what she probably wants to do is WIN AGAINST ANOTHER WOMAN, like the side chick did against her.
You know, I didn't think about that. The idea that she'd be desired even though Rokuro has someone else was exciting and validating because she was doing what was done to her. A well-liked, relatively popular author who's older than her, smooth, romantic, and charming around her, and who has such a beautiful family already? The idea was too much for her to resist and it made her feel desirable again.
When it turns out she was basically making up a story about him finding her so attractive he'd betray his family, that'd bring about the sort of reverse uno effect we saw here because now not
only is she not beating anyone else, the realization that she was basically hoping she was a homewrecker would hit her and re-enforce the idea that she's not worth anyone's time.
My reservations are that husband didn't seem to be all that sexual of a being, but we also haven't had a ton of time with him. He's staying with a bit of a feminine looking art guy, but they just seem like buddies who don't have anything going on. This can be a COMPOUNDED MISUNDERSTANDING where her husband didn't want to marry her because he doesn't actually like the idea of marriage or sex in general, Yukino believing that made her a failure of a woman who was deeply undesirable, and the rumors starting up about an affair confirming that belief too deeply to shake. When the whole time the dude just isn't into, you know, sex. Nor making love. 50 Cent's mind would be blown.
remind me, is his backstory the one where he had an affair with his father's mistress or something long ago? and the love interest's age actually matches to be his child assuming that he did knock up the mistress?
Not only that, but she grew up in the town where the novel that was basically his autobiography he wrote after his father died is set. It's one of the flags that I'm hoping doesn't mean what I think it means, especially because the two look nothing alike and there's never been any comments about how she looks anything like his father's mistress, someone he sure as shit would remember. I also don't believe the age is an exact match, just sort of in the possible range. Which honestly would be anyone in their 20s. This is something that people other than Yukino find upsetting, but she very notably has never reacted negatively to his age and
seems confused when others call him old.
One of the reasons I think this is actually supposed to be, rather than the tragic 'love that shouldn't have been' stories the author has written before. But the possibility is there and I'm pretty sure that knowledge ends in ffffucking suicide for the both of them.