Why do I get so upset when he watches porn?

I asked my fiancée when the last time he watched porn was and he said about 6 weeks ago or so. I promised him I wouldn't get upset. Which I didn't with him but on the inside it kills me! I don't know why! It makes me feel like I'm not good enough and I can't please him enough. I can't stand the fact of him watching another women in an intimate setting. Why do I get so upset over it?!?