This is in no way normal. The fact that what he's seeing excites him puts him one step closer to truly acting on his excitement. Also, it is illegal to have child pornography and if these things are on your computer you could be held responsible legally. At 17, you're young, exploring your bodies and sexuality. THAT is normal. Being 17 and exploring sexuality through children is NOT. I highly recommend you break away from your boyfriend. If something feels wrong, it usually is and you should learn to follow that instinct. You have it for a reason. I wish you the best of luck and smartest of decisions.