<human>: Some info first
I'm Female 22 and he's 23.
Been together almost 3 years
No children or finances together. We also do not live together.

So, while I was getting ready for work at 7:45 this morning, I happened to go through his phone because it lit up. Well, the most recent messages were from one of his ex's. It was mainly a sexual conversation with both sides encouraging the other and she sent him some photos of herself. 
Well, over my lunch, I decided to say something to him about it and he basically made it seem like I shouldn't be upset about it and it *was* my fault for being upset. He said they were just friends and nothing was ever going to happen between. Also that he was just taking advantage of the fact that she was drunk and looking for attention. 
So r/relationships, 
How do I deal with this and how do I set ground rules so that I don't see controlling?

Can you give me a shortened version of the text?
<bot>: My SO of almost 3 years had sexual text messages/pictures from his ex and he responded in the same way so how do I deal with this?