As absurd as his line of thinking is, I can guess at it: he thinks that nearly kissing is "dangerous" because he thinks that Yukino only wants to follow the book--strictly; he didn't, however, pay attention to her not showing an iota of resistance to that incoming kiss.
That said, if it really was Aya, then women do hate each other. I'm willing to bet that she thinks, "If I can't have him, neither can she". This sort of thing isn't something you do to a friend; the very fact that a picture was taken, even with no malice aforethought, opens up the possibility for someone <i>other</i> than the cameraperson and the subjects knowing about that picture. But what friends does Yukino have, huh? One will throw an absurd fuss if she finds out Yukino is involved, and the other will attempt to boost Yukino's man from her--and that very much appears to be her intent.
