read my boyfriend's group chat messages with his buddies and I am a bit disgusted

Me and my boyfriend have been together for 8 months now. He is 33 and I am 26 and we startes talking about marriage and kids after 3 months of dating. He is the first boyfriend I truly love and care about and the first boyfriend my parents like. He just flew me to the U.K. to visit his parents for the first time. Things have almost always going well but something has been bothering me a lot and I dunno if I should just brush it off. He has a group txt with three of his best friends (two of whom are married and one recently got out of a relationship) and they message on that thing every day all day and talk about anything from girls to work, philosophy, sex, business ideas, and basically everything. They send each other naked photos of girls and try to figure out which girl is hot and which has bigger boobs or butt. One time I read some of the messages and it was about me and I had a fight with him (nothing important) and he told me that I misunderstood them and that's why I should never read his messages with his friends. In fact, I think his biggest nightmare is if I went through that group chat. Long story short last night while he was sleeping I went through the group chat and because it is from a year worth of content, I only put in my name as the search term and even though half of it was my bf talking about me and how he has never felt the way he feels with me and how he would never cheat on me and has a lot of feelings, a lot of other things they talked about made a bit disgusted. For example my boyfriend wrote on there about how he hung out with me and my sister and how distracted he was by her boobs and it was making him horny. His friend suggested maybe he should bang her and he said it's wrong to bang the sister but I suppose I would if she would never tell and we were super sneaky. His friends were all saying how they want to bang almost every girl they meet and he said the worse is when you see hot 17yr olds but it's so wrong that in your head you picture them naked. Other things they exchanged was: we should go to swingers clubs but without our girlfriends. These were only 1/4 of shit they exchanged.