with the marriage interview

First off, Nakaya probably moved back to be with his grandmother because he didn't want to be in Shino's way. I don't think Nakaya's relationship with his grandmother was as bad as it first appeared to be. He was just acting like the teenage boy he was while she was obviously a stickler for propriety. He probably felt he didn't have much freedom when living under her roof at the beginning (don't forget he was a boy around the same age Shino was when he got her daughter pregnant), but over the course of the story he matured and probably would've been able to handle her strictness better later on. On her side, she also grew to be more open, even coming to understand that Shino voicing a boy having sex with another boy was professionalism on his part - not everyone would be able to accept that.

As for staying out of Shino's way - he probably wanted to let Shino move in with Tenryuu. It would've been weird though for them to move in together and have Nakaya still there. Nakaya had only met his father for the first time less than a year before. To ask him to move in with another grown man would've been too much. Plus, there was lots of emphasis on Nakaya already being fairly mature and almost an adult. He wanted to develop a relationship with his father, but he didn't need to continue living with him. Their relationship was never going to be a normal father-son relationship, just as depicted by Nakaya asking Shino for them to live just as two people under the same roof. Nakaya wasn't relying on Shino as a father figure but almost more as a respected, adult friend.

Besides, what 17 year old would want to come home to find his father in bed with another man? Or lie in bed awake at night every time his father doesn't come home because it may mean he's at the other guy's place instead? xD;; Moving out could also be seen as protecting himself from having to deal with those kinds of thoughts.

As for the other thing about why Nakaya disapproved so much with Shino being with another man, I think there are a few things to consider. Firstly, over the course of the story Nakaya becomes more open with homosexual relationships, from being disgusted hearing the cds for the first time to ending up in a homosexual relationship himself. I think the main question here is why was he still so strict on Tenryuu even after he himself had been with Fuse? Firstly, I think Nakaya was already acting more leniently towards Tenryuu. He didn't just out right forbid the relationship anymore but instead spoke openly with Tenryuu about it. Secondly, I think the reason he gave the condition of if either fell in love with a woman Tenryuu would break it off is because that's the kind of culture they lived in. It wasn't openly accepted to be a man in a relationship with another man. There are many difficulties in a life where people would scorn Shino and Tenryuu for being in love with each other. Obviously their coworkers were great people to be around since they either joked about or ignored it, but that wouldn't be the majority of people. It wouldn't be surprising if the grandmother tried to keep Nakaya away from Shino again if she found out he was sleeping with another man.

Shino also depicted the kind of culture he was living in with his slowness to accept his own feelings. The thought that there isn't anything beyond the initial relationship with someone of the same sex as opposed to there being a course to follow in a heterosexual relationship is something I've seen touched upon in many BL stories. In a hetero relationship, the two can marry, then look to the future with having kids. Those things are often juxtaposed as 'non existent' in a homo relationship (not my own thoughts on the matter but just what I've seen from reading a lot of BL). 'There is no future in that kind of relationship' - that's the same kind of sentiment I feel is in Nakaya's condition that should either of them fall in love with a woman, Tenryuu will end it. They can be happy together for now, but they won't hold each other back should they find a more socially acceptable happiness, perhaps?

Anyways, the last thing to touch on is the fact that Nakaya himself had been in a homosexual relationship. Even though he'd had that kind of relationship and didn't want to end it, he still wasn't completely open with Tenryuu being with Shino. And I feel that that's again related to happiness and social acceptability. In Nakaya's life, he's still young and figuring out the world. He's not thinking years in the future, even if he says he will (when his girlfriend thought she was pregnant). Shino however, is at the point in his life where he'd want stability in a relationship. He'd be in the relationship for the long haul and having a relationship with another grown man wouldn't be considered experimenting. He was slower to accept the relationship, but that was because once he was in it, he knew he'd be serious about it and wouldn't be able to leave it quickly either. Nakaya probably wanted to protect Shino from the relationship and the position it would put him in and so he wasn't able to accept it wholeheartedly. However, in the end, Nakaya did say he would always be there for Shino so that no matter what happened in the future, Shino would always have him to come back to and therefore Shino would never be alone. Consider it like this: there's something you'd do because you know you can handle it, but if you saw your loved one doing the same thing, you would be nervous and scared for them and would try to stop them.

Anyways, I think that should be it for this extra to my comment xD