You did not know he has a girlfriend. Nobody can tell you what to do, nor do they have a right to. You are the only one who can decide this for yourself.
Here are the things you can ask yourself. If you were the girlfriend, and found out that your boyfriend was kissing another woman, how would you feel?
Did he even think it unethical to kiss another woman while he has a girlfriend? Maybe they have some kind of open arrangement to see other people, in which case it is fine for him to do so. Nevertheless he should have told you if it is so.
Do you want a man who would lie to you, deliberately?
All your answers are within. Trust yourself and you will be fine.