If a man did to two women what Miwa did to these guys, yes. I'd call him a jerk asshole twerp selfish pig.
Of course we would. If this had been three adults having sex without the emotional commitment it would have been fine. But this doesn't represent healthy polyamory.
Yeah. The hate isn't even about the polyamory. It's about how selfish and cruel she was to Shosuke. She didn't confide the truth in him. She used him. And she insulted him. Then she went and played around with another guy. No, she doesn't deserve either guy. What did she sacrifice for Shosuke? Nothing. He sacrificed so much--emotionally, physically--for her.
I see everyone in the comments saying bad things about Miwa but if she was a man with two wives would you guys still say the same things? In the end it all worked out but don't hate her for her position truthfully the situation could have been handled better but it wasn't.