he was with another man this morning

Last night my man of 8months went out for a drink with his workmates for someone's birthday, i was meant to go and join them but I was working til 8 and then had to get straight back home as the neighbors called to say my dog escaped. 
I texted him explaining and he said it had turned into a guys only night and i could go round to his in the morning. I said id bring him some breakfast and left it. 
This morning I bought breakfast on the way and went round to his, i knocked on the door and opened it as it wasnt locked. I got a couple of plates and went into his bedroom to find him underneath doggy style with another man. 
I was in total shock, he looked like he was enjoying it til he noticed me and told me to "get out!" I dropped the breakfast and plates and ran back to my car. 
He's tried to ring me a few times since. I don't know how to handle this. Its bad enough he cheated (so its over anyway) but He has never shown any signs of being attracted to a man or being bi or gay. Also i have some of his workmates on FB and he isn't in any of the photos from last night, not at the meal or in the bars. I don't know how to speak to him about this. Help!