![](https://pic1.mangapicgallery.com/r/avatar/5d/u24030_50x50.jpg?2)
Well, I understood it a little differently. Yeah, he said it was just an excuse, but actually it came from the firm belief that he can't be happy as gay. The uke came from a happy, kind and close family and believed his family would accept him unconditionally, but they didn't. Instead they grew totally apart and that shocked him so much that he believed that being gay there could not be any happy path possible at all. Thus he didn't want to make his beloved unhappy along with him, thinking he still had the choice to be together with a woman as "normal" - without being estranged from his family and treated strangely by his surroundings.
He thought that, if he had been gay all along, the happy normal path wouldn't have been a choice anyway - then it wouldn't have been his fault to deprive the seme of whatever he would lose by being gay. So he rejected him to protect him from becoming unhappy by being gay, totally ignoring him becoming unhappy by not being able to be together with his beloved.
That's why projecting one's own feelings onto someone else is so dangerous ┑( ̄Д  ̄)┍
I don't get it what did the uke mean that he only date gay men when the seme confessed was the seme bi. You know what no one is asking this in the comment section so it was probably already explain in the manga