<human>: Can you provide a brief overview of the text?

So this girl added my boyfriend on Snapchat and kept sending him pics of her cleavage. I didn't really care because I've had guys come onto me and she wasn't asking him out or anything, so I didn't think about it. 

So for a while, my boyfriend would sneak pics of me naked to send to her- claiming that she was bi and he wanted to show me off. I told him to stop, because it was very annoying. He gets a snap of her in shower and she adds that she wants us to have a threesome. I said that I didn't want to be a part of it. I ever I had threesome, I'd never want it with a boyfriend.

A tad bit of back story! My boyfriend goes through my phone all the time and claims its because he felt that something was wrong and I wasn't telling him about something important. 
Well, I broke one of my most important rules and went through his phone for ge first time. He doesn't text this girl, but he snap chats her a lot at work. 

How do I ask him about this? There isn't proof that they're even doing anything, but I just want to know why they haven't stopped talking. He and I have been together for over a year and we live together. I do love him and he's a good person, but this weirds me out.
<bot>: I looked at my boyfriend's snapchat and he hasn't stopped talking to a girl we almost slept with. How do I ask him why?