i'm not sure if anybody else has pointed this out yet but Half man has recurring themes of incest throughout the whole series. I'm not just talking about the half sibling stuff, but the fact that Ruben consistently calls Niall his "son"- which is blood curling once you realize what his dad did to him.
In their relationship dynamic, Niall was a younger version of himself and he was- well, his father. That's why Ruben was capable of over stepping his step brother's boundaries so easily, because he possessed him the same way his dad did him.
Which explains his homophobia (it was ownership all along): Yes, he can have an homosocial incestuous relationship with his stepbrother because that's just family! But if Niall starts delving too far into relationships and adulthood he'll sure come crash his life and bring him back to square one.
That's why back at his college dorm he makes sure to physically intimate with both of his roommates; including the woman who was crushing on Niall. That's also why he beats Albert near dead, so that Niall can prove his loyalty. Ruben knows Niall is gay and he knows it's a matter of time he finds out for himself, so he can only involve himself and make him choose him over anything else.
The biggest betrayal is not that Niall has sex with Mona, but that he spoke the truth during their first trial.
All in all, the reason to Ruben's fixation was because nobody else but him could live out his fantasy, he could only live his sexuality vicariously through Niall.
















