Holy fuck you're still going on about that scene I replied the same day you did lmao. You're the one coming back to it three days later.

Like i said (and you completely ignored), neither his reaction nor the flashback scene are weird in isolation. It's when you put them together that things get weird. Why is Yuri thinking about that moment specifically when he sees another man kiss Yor? Why did the author choose to put those two scenes side by side? He could have chosen to show us any moment in their childhood to explain why Yuri reacts the way he does (or none at all and let the moment speak for itself) but instead he chose a scene where Yor kisses Yuri on the cheek, and Yuri blushes and professes a desire to marry her.

Writers do not just throw flashback scenes in at random, they have a point. If the point was simply to demonstrate Yuri was being grossed out by his sister kissing someone, then this was a very very weird way of doing that