SUBREDDIT: r/relationship_advice
TITLE: I [24F] found my boyfriend [26M] sexting a girl [27F] he hooked up with 4 years ago.
POST: I was playing with my bf's phone when I accidentally opened his Facebook messenger. Out of curiosity, I opened the first one, which was a conversation between him and the girl he hooked up with in Thailand when he studied abroad there in 2011. I have no issues about my boyfriend talking to his exes, and it does not  threaten me much since this girl is still in Thailand and my boyfriend often makes fun of her. However, it did bother me when I read the stuff he was messaging him, like what he would do to her sexually if he was with her in Thailand or something. I know he is not emotionally cheating, since he doesn't like the girl. But based on the message thread, he was the one who started the dirty talk. I have not confronted him yet and I don't know whether to let this slide or check the messages again when we meet in a couple of weeks then confront him about it. And if I do, I don't know how without making it sound like I snooped through his phone. Any advice?
TL;DR: 
saw some dirty messages on bf's phone to this girl he hooked up with four years ago who is a thousand miles away. Should I check again next time and how do I confront him about it?