The answer is NOT what you expect.
My gut temptation is always to slap them across the face, but since that would ultimately not be helpful and because violence is never the answer (pacifist 4 LYFE, son) I tend to blow their minds a little bit when I tell them what I really think:
"Well, it depends."
Don't worry, I'm not about to waste your time rationalizing cheating. But I do think relationships are varied enough and complex enough that issuing one decree about what's permissible in all of them is kind of short sighted and lame.
The only rules in a relationship are the ones agreed upon by the people IN that relationship. Period. End of sentence.
For some people, kissing someone else might be no big deal, heck, putting a finger up someone else's butt might not be a big deal to some people. It's really up to the people inside that relationship.
Kissing is regarded by many as an intimate act. I know because if someone on the street just darted up and tried to kiss me on the mouth I'd probably puke and run away.
I also know that because kissing is the one thing Julia Roberts' whore character in Pretty Woman wouldn't do because she was like "nope, too personal, look at my beautiful hair and teeth." But I digress.
Simply put, if the idea of your partner kissing someone other than you makes you uncomfortable and angry or kind of want to poop your pants, that's fine. Tell them that.
Make that one of your rules. If you make explicit rules, then you don't need to worry about someone "not knowing" something like kissing was even against them, to begin with.
I'm dating a polyamorous man, but that doesn't mean we both get to do whatever we want. It means we have to talk to each other but what we want and make sure the other person is okay with exploring those wants and desires.
And guess what? Sometimes one of us isn't, and that means that kissing, sex, and butt touching all go off the table.
Your relationship, your rules.
If you find yourself asking "is it okay for me to kiss this person? Is that cheating?" The bigger questions you might want to ask yourself is what are you looking for outside of your relationship?
Are you happy? Are you looking to be unfaithful? Would it stop at kissing? Because you know if you're asking yourself or a stranger that you're probably already doing something violates the agreement that you and your partner have established.