Read chapters 1-2, 4.
Summary: Inexperienced, 'straight' MC who is fooled by a friend into taking on his debt. He is helped out by the ML, a gang member who essentially covers the 5 million? Debt in exchange for the MC's body. The MC tops, while ML bottoms and is quite assertive about it, and MC can't resist. Dubious consent since MC was slightly manipulated into it.
TRIGGER WARNING: GANG RAPE. (CHAPTER 3) it is completely skippable. Go straight to chapter 4. I don't know why it was thrown in the story like that, so randomly.
Yakuza-sama no Omocha