In my opinion...

In my opinion, the seme has always liked the uke, but never realized that his feelings were reciprocated, so he preferred not to interfere with their friendship because, if he confessed, (in his mind) the uke would possibly distance himself and he would lose the only person he has a sincere friendship with. Just look at how hurt he was when the uke went to the army and lost contact for so long. Let's remember that the seme never forced the current situation to happen; it was an accident that ended in their sexual involvement. The seme probably doesn't even know that the uke was a virgin – and maybe didn't even know he was an omega. It seems to me that the seme still believes that the uke only likes him as a friend and is afraid that he will leave after their pheromones subside, which is why he insisted that he promise. In other words, in my opinion, the indications are that everything is just a misunderstanding and the seme secretly loves the uke, while the uke also secretly loves the seme.