<human>: So a few days ago I was using my boyfriend's computer and found a bunch of searches for a photography studio specializing in boudoir shoots. It's located in a completely different state, so I knew he wasn't looking at it for my benefit. The specificity of the search made me question why he was looking at this one place. 

So I did something I'm not proud of, and looked at his texts. I noticed a conversation from that day with his ex, who lives in the state where the photography studio is. She had apparently had a shoot there, and sent him a screenshot of a picture off the studio's Instagram page. In the photo, she's in a bra and underwear, and the bra is clearly unhooked. 

He didn't respond in a way that makes me suspicious of him at all - I'm not worried he's cheating. However, I don't feel that what she did was appropriate at all and it worries me even more that he didn't show me the picture. We have a rule that if it would hurt or bother the other person, we won't do it, say it, etc, no matter if the other person will see it or not. I KNOW he knows that this would not be okay with me. I'm also bothered that he searched for the photography site. I feel like he was looking for more pictures of her, and that makes me sick.

How do I bring this up to him? I really don't want to tell him I looked at his text messages but it may be unavoidable. I don't think he's cheating, I just want him to realize that this stuff is NOT okay, and I don't know how to bring this up to him. How do I let him know that if stuff like this happens, he needs to tell me? I know if I bring it up, he'll focus on the fact that I looked through his texts.

Can you condense the text into three sentences or less?
<bot>: Ex sent my boyfriend a picture from a sexy photo shoot, I know he isn't cheating, but I don't know how to lay the ground rules for stuff like this.