I wouldn't be happy if a boyfriend of mine flirted with someone else, but it's such an imprecise term that it's hard to say. I wouldn't consider it cheating, just a bit rude, especially if I was there. Kissing is definitely cheating, but is forgivable. Actually, I'd forgive most things if I liked someone enough, but only if they genuinely were sorry. It's hard to say, because I've never been in a really bad situation of this sort. It's the sort of thing that would really upset me; I still feel a little indignant when I see ex's kissing someone else, even though I know it's ridiculous.