A spineless weak willed MC is the hallmark of almost every "reverse ntr" story (I would know, I eat that shit for breakfast), otherwise they wouldn't allow themselves to be banging other people and the plot wouldn't move forward. The entire shtick (ie where the pleasure comes from) is the guilt and regret they feel for being so pathetic and weak to someone else's advances, and the betrayal they have committed towards their loved one. They have to be susceptible to seduction, or in this case manipulation.
If you're expecting this man to change... You're looking at the wrong manga. This is just an outlet for the author's netorase fetish.