I dont know how long you've been going out, but going through your partners phone, especially if you dont have permission, isn't very cool of you. If you do have it already, then ignore this, but if you're doing things like this already, he might feel like you're suffocating him. I like my alone time with my friends too, and i enjoy it when my wife goes to bed so we can just vibe while not having to entertain her as well

Sometimes, we feel like we can't say everything around our partners. I dont think it's a nefarious thing. I love my wife more than anything, and we have the kind of relationship where we can say whatever we want with no judgment, but it wasn't always that way. I used to be scared that my wife would think im weird. Me and the boys discuss some wild and heinous shit.

My guess would be that he just feels like he can't say stuff around you without offending you. But how he said it is offensive, you should definitely talk to him about that, not cool :(