sorry while i agree somewhat...i would have the same reaction too. He has gone out of his way to plan dates and be there for him, he has not once looked at anyone else or implied he was...in fact he had told him he loves him a few times but as soon as marriage is mentioned its automatically assumed that thats what he wanted because he dated girls in the past? As for the insecurity i do get that but in what actions has he shown that its something that would happen? i would be the same and question whether you thought my love for you is real or if me dating a girl in the past is what matters now.
i think just the fact that even the word marriage shock him means his insecurity is that deep and it probably stems from the fact that he has love saburou all along while saburou has dated women in the past and although that dont matter it is still a thing he mull over and they probably just need to reassure each other until such feelings is diminished but im just a bit eh when saburou treat his insecurity as an annoyance rather than reassure him and yeahh the uke is overreacting , he shouldve just said why he was so bothered rather than lashing out
imo,yuuji has every right to be insecure,did he overreact? yeah but its only bcs hes been in an unrequited love for so long towards saburou all the while he was dating other girls and whatnot ,if you're in a relationship with someone you should talk gently with them about their insecurities in the relationship rather than finding it stupid just bcs you dont think much of it than your partner do,,that dogeza was so unnecessary and i know its just for comedic relief but it just goes to show how much yuuji love saburou and how insecure he is about the relationship,,saburou shouldve been a gentleman and remind yuuji how much he love him instead of brushing it off as something stupid （｀ェ´）