@anonymous1896,
Quote:I'm more worried about how to deal with the guilt I'm feeling
Like everybody else, I find surprising that you would feel so guilty about such trivial matter, although I'm not entirely clear what "peck" means in this context. Not a native speaker, sorry. Does it mean you kissed that boy on the cheek, or on the mouth (closed)?
Kissing on the cheek is what friends do to say hello, where I'm from. But that's not where you're from. Guess you speak from the perspective of someone with much higher moral standards about cheating than mine. You broke your own standards, and you feel bad about it.
You are right about that. The only moral perspective that counts in this case is yours.
Two options come to mind: one is to speak to your BF about it. That's risky but the "right" way to go. It could work, especially if your BF is a good man. He could laugh the whole matter off (as he should) and relieve you of your guilt feeling. It could also backfire spectacularly and ruin everything. The wisest advice on the matter is "N'avouez jamais!", I believe.
Another option is to lower your standards, not necessarily in the way you assess your own behaviour, but at least when judging others. You could go easy on judging your BF about his own supposed "trangressions" of your norms, for instance.
That'd be in application of the saying: "Let (s)he who's without sin throw the first stone".