Was I right to leave my boyfriend over this?

Well... you did tell him that it was okay to see other people. I know that whenever we say this we don't really mean it, but if you don't really mean it then don't say it. Also, he could've just not told you and everything would still be peachy between the two of you, so the fact that he did tell you means he really cares about you. I know it must have hurt you, but you should own up to some of it. Besides, it it's his first serious relationship, he is bound to make mistakes. Whether you're willing to put up with them or not is entirely up to you, but what you can't do is expect him to act as consequently as you, who have had a fair share of serious relationships. If he is worth the trouble, and you think he will truly change, then give him a chance :)