I don't understand my boyfriend

I was on his phone, but not because I was going through it thinking I couldn't trust him or anything. What I found was by accident. I had seen a search that he made wondering if there were alternative applications like craigslist casual personals. I didn't quite know what that meant until I looked it up myself. Then I didn't know if he was interested in cheating on me and hooking up with other people in his area, and I also didn't know if he didn't enjoy our current sex life. 

(it's not like we have a bad sex life, I haven't picked up any vibes of him not enjoying himself. And we have sex frequently)

Then I found the craigslist app on his phone and decided to look at it. I'm not sure how the apps work but there were favorites under a tab, and when I looked under a tab, I could see the saved searches and they were all male for male casual personals. In my state, prostitution is illegal so everything that was on there was strictly just for a free hook up, no money required—no money involved. I know that he has a funny personality and I didn't know if he just had that to read funny personals that people put out. I also had no thought about him being gay or bisexual or wanting to experiment with that. Then I had seen other searches for people looking for threesomes. 

He doesn't know I've seen this and I don't know if he is still active on the application, but I don't know what to do about it at this point. I don't want him to know that I saw that, and I don't know a good way to bring it up to him.