If you weren’t even calling it a relationship yet, then it certainly wasn’t exclusive. Like you said, you just thought it was. That doesn’t mean it was acceptable behavior, but it does put in in a grey area. Here’s something to ask yourself: Are you devastated that your boyfriend’s dick was entering another vagina concurrently with yours? Or are you devastated that your boyfriend may have engaged in willful deception early on in your relationship? One is a manifestation of petty jealousy, and you should just get over it in time. The other is a legitimate concern for what may have been a breach of integrity.
Ultimately, the question you need to answer is do you trust your boyfriend now, within the context of your long-term relationship? If you think you can trust him, then stick around, be pissed for a little while, and then get over it. If you don’t think you can trust him, either find a way to repair that trust, or get the fuck out of the relationship.
Don’t make this about some girl your boyfriend used to fuck. Make it about your boyfriend’s integrity as it applies to your current relationship.