
It's because the tone is rather realistic that the old guy ended up having a relationship with the gay guy and not his straight son-in-law. Having a good chemistry is nice and all but that doesn't make a straight man gay. I would've been extremely disappointed if this had turned out to be another "I'm not gay but if it's you" type of story.
It's also quite realistic that a teenager falls in love with someone quickly.

Thanks. Your comment actually helped me to put a finger on why I like this one.
Among other things it IS good it´s not "I am not gay but..."
Anything you could recommend with a realistic touch to it? It is so.. annoying to be looking for something like that and going through piles of similar yaoi stuff. :(

I don't have that many realistic BL manga/comics I can recommend to you because I usually read dark/dramatic stories or comedies and those aren't very realistic but maybe you'll like one of these (all of them except for one are finished):
http://www.mangago.zone/read-manga/all_about_lust/?av=1
More of a comedy and a little bit smutty but the characters act mature and the relationships are healthy.
http://www.mangago.zone/read-manga/on_doorstep/
Another one with a big age-gap (although not as visible) but rather short.
http://www.mangago.zone/read-manga/shimanami_tasogare/
It's tagged as shounen ai but I don't think it was sold as BL. Anyway, it's pretty good :) .
Then there's the still unfinished
http://www.mangago.zone/read-manga/itoshi_no_nekokke/
I really like it (one of my favorites) but there's a character labeled as straight who ended up in a relationship with a man. However, that character didn't have much of a sex-drive to begin with and their journey is very well written that's why it never feels extremely unrealistic.
I can also recommend
http://www.mangago.zone/read-manga/momentum_park_ji_yeon/?av=1
and
http://www.mangago.zone/read-manga/dark_heaven/
to you but I guess you already know them (if not go read Dark Heaven, it truly deserves all the praise it gets).
That's all I can think of right now :) .

Thanks so much... I will def have a look at those.. I liked 'On the doorstep' and 'Momentum' is...kind of a masterpiece...IMHO...
On the doorstep... I like when the characters look mature.. The adult world.. It's hard to find..
I opened Dark heaven a few times and put it aside, I will try again... I put aside Momentum a few times, then I read a few pages more one evening..... and..finished in the morning, totally wrecked and..it's really good...

Thanks so much... I will def have a look at those.. I liked 'On the doorstep' and 'Momentum' is...kind of a masterpiece...IMHO...
On the doorstep... I like when the characters look mature.. The adult world.. It's hard to find..
I opened Dark heaven a few times and put it aside, I will try again... I put aside Momentum a few times, then I read a few pages more one evening..... and..finished in the morning, totally wrecked and..it's really good...
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Sensei and granpa has more chemistry in them rather than student-kun. They just have more depth with each other. Kouhai-kun's side is too simple for me, it's like a shoujo type where A suddenly goes dokidoki for B, then asking B to go out with him..idk im not feeling them. Considering the vibe of this manga has realistic tone and text, i was sure and hoping that the grandpa will pick the sensei..buut i guess not. Kinda disappointed.