It seems your boyfriend is either being particularly understanding, and doesn’t see a drunken kiss as cheating or worth ending a relationship over, or he possibly kissed some women in a club, is afraid you might find out, and is trying to soften the blow by telling you that if it was the other way around, he’d be cool with it. If it’s a case that he has done this, given he’s telling you he would be fine with it if it was you that did this, he might think that you’ll feel the expectation to feel the same way.
What was the context of him saying this? Did he say it out of nowhere, or where you having a conversation about cheating, or along similar lines?