Hear me out, guys:

Theory: I don’t think that was their first kiss. It was just their first onscreen kiss.

In the scene where Yuuri was crying, Victor asked him if he should kiss him, quite casually too.

We’ve seen that technique to stop crying successfully twice in Junjou Romantica, a BL anime, as seen here. Indeed, both those times were the first kisses of the couples in JR, so why would I think this was not Victor and Yuuri’s first kiss?

Because of how Yuuri reacted. He said no, and then:

He’s not even surprised at the fact that Victor just proposed a kiss. There is no disgust in his response; in fact, he goes on to explain why that isn’t a comforting move right now. That could possibly mean that Victor has asked to kiss Yuuri before. (I wonder why…)

Now some of you might say “well, wasn’t Yuuri surprised when Victor kissed him after the performance?” and yes he was. But I think that was because they kissed in front of the whole world, literally. The Grand Prix is probably viewed by millions around the globe. Yuuri, a very private person with some level of anxiety, is not our top choice for someone who indulges in PDA. So no matter what proclamations he makes about Victor, he’s not one to kiss the guy in public. The fact that they did kiss in public was the surprise, in my opinion, because, after the initial shock, Yuuri has a peaceful look, and he doesn’t even question the kiss. He just says:

And returns Victor’s loving, understanding gaze with one of his own.

We have seen Yuuri get flustered at lesser things (such as being asked to be Victor’s lover), If it was their first kiss, he would be more shocked, and for a longer time, than he was.