Steve blinked, and for a moment didn't seem to fully comprehend what Tony had said, or what he had really just done. Then realization seemed to settle upon him, and if he hadn't been Captain America one might have mistaken that small turn of his head as a way to hide the faint blush that had grown on his cheeks. His face felt hot, and the right words didn't seem to know how to make it off his tongue.
He really shouldn't have done that. That wasn't a friend thing. That wasn't appropriate, and especially not now given that despite it's embarrassing nature, the tale of Black Cat was an important one to tell. This led one part of his mind down the path of when exactly it was appropriate to kiss your best friend, and he prompt shut that voice up, that was a whole other can of worms.
"...I'm...sorry, I wasn't thinking...I...someone's perfected the Super Soldier Serum." Just dive right in. That would be the easiest way to explain it.
Clearly. New super soldier in town....so I kissed you.