My boyfriend has had sex and I haven't

My boyfriend had sex before we started dating and I'm really trying not to be bothered by it, but I really am. He was super open and honest with me about it and told me how it happened. He has said that he regrets it happening, that it was a stupid decision, and that it meant nothing. It happened almost two years ago, but he still beats himself up about it. It was a one time thing. He went over to one of his friends house and she had been drinking. He kept trying to leave, but she convinced him to stay and he started drinking too. Both of them were underage, but her parents weren't home. He said she kept coming on to him and he kept saying no, but by the time they ended up having sex, he was super drunk. He said because of the alcohol he doesn't remember most of itand that he hasn't done anything like that since. I'm trying to just let it go, but I don't know how. This is my first relationship. I've never done anything with anyone. I don't want to be bothered by it because I know he beats himself up about it, but I don't know how to feel at ease about the situation.