While I don't think Kiyo made the right decision, it's very much an understandable one. She didn't want to be ostracized by her family, and she wanted a sense of meaning and purpose in her work, and for that to be recognized by society. Being a wife and a mother provided that. It just didn't make her happy.
Also, it's not like Kiyo owes Micchan a relationship. She's got a right to make her own decisions in life, even if they're the wrong ones. Micchan also crosses some boundaries at the end of ch. 4 and the start of ch. 5. She says, "you're still mine", as if Kiyo is in some way her possession, and tries to initiate sex Kiyo is clearly not into.
Poor Micchan, what a trash first love of her. I don’t know why Kiyo-chan is so shameless showing her face and go on trips, killing the mood of telling she had sex with her fiancé and unknowingly got pregnant. Fucking slut. But well, she got what she deserves choosing an asshole husband fucking with a long time affair and while she sulking in pain of her wrong decision.
Slut shaming is sexist, please don't do that.
Furthermore, Kiyo wasn't "killing the mood". Micchan wanted to have sex and Kiyo didn't.