He was scared of breaking Akira. He didn't want to ruin what they already had but at the same time he didn't want anyone else to have him either. Seiji loved Akira but was terrified of being rejected. That's why he disappeared when Akira said that he didn't want to have sex anymore. He thought Akira was sick of him.
Seiji became too obsessed with keeping his silver spoon clean and perfect that he went on and sullied other spoons instead. It’s a pretty awful thing to do esp since he’s doing it with no real feelings whatsoever for the girls and he just does it whenever he feels the hots for Aki.
I see I understand now but yeah like you said it is awful and heartbreaking at the same time.
I dont understand why the older brother sleeps with other women just for the sake of his little brother . Arrrgh like its totally unnessesary for the plot . I only like Satoshi in this whole series