She was a hopeful, skilled swordsman trying to advertise her sword style. MC pops up, "ay gimme dat style." and spends time with her, leads her on, and she falls in love. MC says "aight, lol, seeya." in a way that convinces her that she isn't worthy of him.
MC didn't sudden leave, he had to leave because of circumstances surrounding the Elves. And Kaeha understood it completely (she never considered herself as unworthy).
The problem was the Careless Words MC left her with, which acted like a Curse on her.
If the Elves situation hadn't occured, maybe Kaeha would have eventually confessed to MC and maybe he would have either accepted it or rejected it. Both ways Kaeha had the opportunity to eventually move along with those situations.
She then loses her mind after he leaves her and has her friends husband inseminate her (ruining their marriage) so she could produce heirs worthy of the MC.
Well technically, Kaeha was friends more with Creias (aka.,the husband), than he was with Martena (aka., the wife).
Because Creias was the one who helped the situation with the opposing Dojo, while the only man she knew who was the strongest (as she had a bout with him, and later had help from him during her Adventurer work days).
To Kaeha, who didn't know any other Man nor had the will to marry any other man (as she loved MC), she had no other choice than to go for his help even if it was a murky situation.
And i wouldn't really say that Kaeha ruined the marriage between Creias & Martena.
Both understood the implication of their decision, and in a way i would say that it was a better solution, as no one betrayed anyone else by keeping things hidden or in secret.
And if their marriage was really ruined, then they wouldn't have stayed happily with each other for so long and died soon after eachother (they say that, the stronger you love someone the harder it is to live after their death, and that's why it's common for people in their old age to die soon after their loved one had died).
