Need some advice..

My fiancé and spent the weekend doing blow. Friday night he sat out in the backyard on the patio away from me for most of the night.. I was a little upset about it but I just watched a movie by myself. On Sunday i came outside and saw that he was standing at the side of our house on his phone, there was a lot of noise going on at that time so I quietly creeped up on him to see what he was doing since he looked a little fishy. When I looked over his shoulder I saw he was on some website where It looked like he was on some chicks profile with a picture of her with her boobs out. He quickly closed the Internet and my natural reaction was to turn around and walk away. He walked to the front of the house while I went to the backyard. I sat there for a second not really 100% sure what I saw but was hoping it wasn't something very bad. I  decided to walk back to him and I asked him what the fuck that was. He replied with "it's nothing" and "it was just a little porn I watched a couple days ago and I was deleting it before you see it". When I asked if it was really just porn or was he actually cheating he said "shut the fuck up" and walked away. I don't know what to think of it. Im pretty sure I saw him on some skinny chicks profile with her naked body where you actually talk to these women NOT just regular porn. I don't know what to do.. I don't trust him at all anymore.. Is he cheating on me or was it really just a little bit of porn? What do you ladies think? How do I tell if he's cheating? What also REALLY upsets me is we NEVER fuck anymore (we used to like every single day) and now he doesn't even touch me but he will watch porn?? Ugh...