My [22f] boyfriend [25m] cheated on me because of a "joke". How serious is this?

So my boyfriend literally came home from work, bragging about this "funny story" he had that he "couldn't wait to tell me". And he told me about one of the women he works with, who apparently was flirting with him all day, and for some reason, decided to tell him that she forgot her underwear that day. And so he and the woman were joking about how he was going to check if this was true. Well, he said that this woman literally pulled her skirt up and showed him her bare ass. He admitted to me that he touched her and then the woman kissed him and they took a selfie together. He said nothing sexual happened beyond that. I confronted him and he tried to say that this woman was just trying to "get a rise" out of me since she knows about us. I don't know how seriously to take this. He swears that nothing serious happened but the fact that he went along with this woman's antics and actually engaged her in something sexual is just so disrespectful in my opinion. How bad is this? Should I address it with this woman? Do I even believe my boyfriend's story since he was so clearly trying to hide this from me until he decided to actually bring it up to me?