Why did my boyfriend go to a strip joint?

Last Saturday my brothers, cousins and boyfriend went on a stag night. I knew they were going to a strip joint but I did not expect my boyfriend to get a private dance.

The dance apparently was behind curtains and ever since he told me (the morning after) we have not stopped arguing.

Although I put all my trust into him going I feel he has betrayed me by getting intimate with one of the dancers. Also when questioning him, I asked him to swear on my life that everything he told me was the truth. He did.

Then I noticed that his hands were behind his head so I asked him again to remove his hands and to say that he swears that he is not holding anything back from me about Saturday night.

He replied that he could not do that, but that he would swear on his own life. Is he telling the truth?