Responses
Natural possessive instinct. Animals fit to survive have it. Also he is jealous because he's loved Izumi for 15 years but was stupid and someone else got to Izumi's butt first so that adds more to his frustration and regret for not being able to just man up and tell Izumi his feelings since the beginning.
I wished Izumi and the bro would be together though. And it's impossible Izumi didn't felt his ass stinging the next day after he had sex with the brother-in-law. He just made himself believe thatvhe forgot but I'm sure he didn't. Shin is...... I don't know. A bit pushy, he dated and had sex with men before so why is he so frustrated when Izumi slept with the brother-in-law once?