Found an old video of my boyfriend having sex with another man. I really need some advice.

Ive been with my current boyfriend for a year. He has never shown any signs of being sexually interested in other men. Our sex life is fantastic, and he shows me more than enough attention in that area. The other day I was looking through some of his files (I know this was wrong, but lets save that for another day). While doing so, I found a video that is clearly from when he was younger (he was much skinnier in the video which tells me it must have been at least 5 years old). Im not trying to be graphic, but the video showed him engaging in sexual activities with another man (i.e. being penetrated, giving oral-and doing a great job i might add, and also penetrating the man).The video was shot POV style, so I only knew it was him from the oral scene. He seemed to be enjoying it. Also, the person he was doing it will had fake nails, so Im assuming he was, at the very least, on the feminine side. I tried to bring up a topic that would allow him to give me some indication of whether or not he has ever been curious about men, and he didnt take the bait. I am concerned that bringing this up could result in a huge fight or a break up due to the fact that it is so private and I was snooping through his things.I havent found another ounce of evidence that this was more than a one-time occurance, and he has no gay porn or pictures either.I would really like some advice about what this could mean. Is he bisexual, or was it just exploration? That video keeps playing back in my head, and I really want to understand. I am pretty open-minded, but I just want to know what Im dealing with here.Constructive feedback is welcomed. Thanks!