To me, it's never ok, not in any way shape or form. I broke up with my first gf of two years because she kissed someone. To me, it's the ultimate slap in the face and it's inexcusable. I still loved her at the time but I couldn't trust her, and she knew how strongly I feel about that sort of thing.

And I honestly think you should tell your boyfriend, because imagine if he cheated on you and didn't tell you, it wouldn't be fair.