Why do guys hide things from their girlfriends?

I recently discovered a photo of my boyfriend licking another woman's face. The photos were from 10 months ago when he went to the races with the boys and had a big day/night. At the time we had been dating 6 months and I was 6hrs away in another city for a conference. He had called me that night and when I saw him next he uploaded the photos from that day/night onto my laptop but obviously he had deleted some photos. When I recently saw the photo I said "You right there licking her face?" and the reply was "yup". So I then looked at the rest of the photos and found numerous photos of him and his mates with this chick that he had failed to upload onto my laptop. He had his head resting on her boobs in one and was licking her face in another. We talked about it and I later found out that the girl tried to kiss him but he refused her advance and so she moved onto one of the other boys. He said he deleted the photos and didn't tell me because I would get upset and over react. We have both been cheated on in previous relationships and I found it bizarre that he would flirt with a girl so much so for her to try and kiss him! He claims that everyone was flirting with her but none of the photos show his mates flirting or resting their heads on her t*ts and licking her face. I have told him the fact that he hid it from me makes it worse if he was upfront and showed me and talked about it I would have been upset but nowhere near what I was when I found out. I have told him that sort of "flirting" is one step too far and I don't accept that behavior. Am I over reacting? Should I worry that he won't tell me the truth about other things? Should I worry every time he has a boys night? Why hide that from me?