Do I really look like the sorta guy that runs around just kissing people?

[ Izuku has to know that Katsuki has never been interested in romantic overtures. Has never accepted a love letter. Chocolate. Valentine. Nothing.

So him kissing Izuku, being the one to initiate it... That's a big deal.

Almost a big of one as what Izuku says next. ]

Pretty sure you'd have to kiss me yourself before you can do it again.

[ Implying he had kissed Izuku. Not the other way around. ]