he's young, he's full of hormones and testosterone. forgive him for his stupidity, lol. it's just another avenue for photo porn and the like button is just too easy to click for him.
I agree with this...he's young and stupid. If you really care about him and want to solve this problem maturely, tell him how you feel...and tell him that you're not angry about it and it's not that you feel like he's a cheater etc....but tell him that you just feel hurt and that you'd appreciate it if he could stop. But if you come across as angry and cold, you may push him away rather than bring him closer.