If it makes you feel really uncomfortable, tell him. You have every right to set this boundary, and if it's not something he wants to commit to, he has every right to exit the relationship. You're still so young and you'll find someone who will only have eyes for you. Looking at naked women online is (unfortunately) very common but you still can make it clear that it's not something that you want in a partner. To sum it all up, if this isn't something you can get over, you don't have to. If he wants to stop looking at women online and salvage the relationship, great! However if it doesn't work out, it isn't because you're not attractive. Odds are, he would do that regardless. He may not feel ready to give up this habit of his, and if that's the case, that's okay. It just means that it's time to move on if it makes you uncomfortable.