"MPH-"
His eyes widened for like-a good ol' 10 seconds, and he didn't kiss back after like-15 seconds-but after those 15, he finally kissed back, eyes lowering as his hands wrapped around his neck, melting into him slightly...now, here's the thing.
While he was with Ren, he never liked physical touching. To be honest, the only reason why he was in that relationship with Renin the first place was because of his blood-he didn't love him at ALL. Ren was just another student at UA him-he didn't like him at all. I mean at ALL. And when he slowly started growing more and more, his addiction to going to parties became stronger-and he'd bring Katsuki along for somewhat reason, usually leaving him for the night and he wakes up he has a whole ass stranger on top of him-and when he'd look around, well, Ren would simply be...gone. Of course, he wasn't surprised, but it just...made him have more of a reason to leave Ren for someone else...but, he simply couldn't.
Kirishima on the other hand though, well, to be honest at this point, Katsuki wasn't even thinking about Ren in the first place, if he was, he wouldn't be kissing Kirishima right now. He really wouldn't-he'd probably pull away and just forget about it in a couple of days...oh but no-this wasn't something he was going to forget so easily because it was with someone let's just say...he "cared" about as Kirishima likes to say it-
(Sorry I wrote a lot I'm just right now in the writing mood-hehehheheheh-)