We dont know if this is the first time he's been overtly physically abusive. It's just the first time we've seen it. Either way the guy has been abusive from the start. Emotional and psychological abuse doesn't always take the form of outright horrible screaming and obvious vile denigration; it can be more subtle, and often takes the form of gaslighting. This isn't me speaking as an armchair psychologist; this is me speaking from firsthand experience of that kind of abuse that dominated the first 3/4 of my life.
I've had 0 sympathy for Tazune from the beginning due to my own life's lessons and shitty history, and nothing but sympathy for Midori despite her confusing and hurtful behavior for the same reason--it mirrors my own. But that aside this kind of experience is all too common and I think the author is doing a great job of depicting these kinds of wretched dynamics and situations. It's a definite deviation from the typical yuri we see, which are typically framed from a younger perspective--which is also perfectly valid, but only part of a greater picture that is so unfortunately common once adulthood is reached. We're getting a much less common depiction of what can often happen after someone "graduates" into adulthood and all its potentially poisonous pitfalls with regard to homosexuality and heteronormativity.
Buuuuuut, I would've hated Tazune if this was a het story, or a yaoi story, or if Tazune had also been female. An abuser is an abuser regardless of gender. Ditto for victims of abuse. Anyone abusing anyone is reprehensible, but hitting a pregnant woman is worse. Even if she didn't get out for her own sake, I think Midori would get out for the sake of the kid. After all, so far "I'm not one person anymore, I'm two" has been a pretty big theme in her thinking.
I hope Maki confronts her about her behavior and they both get some closure and growth, and I think the author is capable of that. But we'll see.