Read again. I don't think it's double standard but rather I think it's Kengo personality to sort of coldly analyze the problem then make his own conclusion. He knows that physical abuse is wrong but he also analyzing why it occure in the first place. He even did that to Hideyoshi (the uke) where he conclude the reason why Hideyoshi got beaten. From what I read Kengo conclude that it's because Hideyoshi is insecure, doubting his exbf feelings (in other words not trusting his exbf), too independent, stubborn, etc. Despite all of Hideyoshi flaws it still wrong to beat him. The same thing apply to the girl, Kengo still think that the abuse is wrong but, again he analyze the possible reasons and from the woman explanation and her overly friendly attitude towards Kengo, he made his own conclusion that she might give mixed signals to other men that caused his husband to be jealous.
I really liked the story the only thing that has me concerned is that I feel like the ukes psychological issues weren't resolved but other than that I really liked the story.