The way I understood it is that the big secret the dark haired guy couldnt tell his lover was that his adopted father was in love with his biological father, which would cause his lover pain because he was estraged from his adopted father for the reason that he himself was gay.
In other words it would be tough to tell his lover his adopted dad was in love with a guy, when his lover and his adopted father werent on speaking terms because his lover is gay.
ummmm, no, that's not it at all.
The adopted son, found a picture of the adopted father and his mother, so there was an impression that the adopted father may have been the real father, and I don't think he wanted to take that illusion away from the adopted son. Because it wasn't true, as he was in love with the husband and not the wife.
So did the dark haired guy like his dad?