You're right, but remember Uenoyama was the one who initiated the relationship with a kiss. I do think Mafuyu should've taken time to work through his issues before getting into a relationship with Ue, but if he just ignored the kiss that would make Ue feel even worse and it would make the fandom upset. If he asked Ue for more time, the fandom would still be upset because they would want Mafuyu to hurry up and get over his feelings for Yuki.
It's important to understand that Yuki was not only Mafuyu's first love, but his first eveything. He was Mafuyu's world, the only person he thought could truly understand him. Not to mention he's a victim of abuse which is a big part of the reason he's so closed off.
Mafuyu is trying, but it's not as easy as saying " I shouldn't compare these two" or "I should get over Yuki" and then moving on. It takes a lot of effort and we need to understand this is only Mafuyu's second relationship. The kid's only seventeen, so he's going to make mistakes.
I love Ue, but I think every character deserves love in this series.
For those ppl commenting that Mafuyu still needed time to move on from Yuki's death, I agree to all of you. But isn't it bad to be in a relationship with someone who genuinely loves you when you're still in the peek of your grieving? I mean, yep Mafuyu still need this, need that, dont pressure him and like that, but as Uenoyama's boyfriend isn't it sickening to always compare and see your late boyfriend with the new one? Sorry to say this but I really really ship MafuYama but I still didn't like how their relationship started with Mafuyu still stuck in his past :(( BUT, hopefully Uenoyama won't snap his thread of patience in understanding Mafuyu even though he always sees Yuki in him. Hopefully, again, that Uenoyama can improve more and save our precious Mafuyu.
Anddd for those who are attacking my beloved Uenoyama, I just want to remind you that this is his FIRST RELATIONSHIP with someone (the virgin one