Every guy would get jealous if you kiss any other boy on his lips.
You haven't mentioned where you actually kissed, but as he is pissed off I guess you kissed on the lips.
Now think that your bf is kissing his cousin...Passionately on the lips. Will you like it ? I don't think you would.
I always say, put yourself in the position of the other person and think, you always get the answer to any question.
For me, I think you should leave your cousin and be with your boyfriend , or at least stop kissing your cousin. One more thing, if you keep allowing your cousin to kiss you, he may get a wrong impression that she is ready to do anything and may take advantage of you.
Stay Away from kissing your Cousin.