I need some advice on relationship/sex as well as some advice about my boyfriend's friend

Hey guys,I need some advice about my sex life with my boyfriend, which may raise larger questions about our relationship as well as a situation with one of his friends.My boyfriend and I have been together for nearly ten months and we're very much in love with each other. As of recently I've found that I haven't been wanting to have much of any kind of sexual interactions with him. When I'm horny and I want sex, I turn to him, but I've found that I can't even keep it up with him, however I do perfectly fine when I masturbate when he's away (we live together).Over the past few months he's broken my trust a few times. I've caught him sexting other men, one of my friends caught him making out with some dude at the bar, and I've also caught him texting guys he "used to talk to". All of these things he's initially lied to me about, but has eventually admitted to them. I've forgiven him for all of those things that have happened. After the last time is when I noticed my sexual attraction toward him to start to fade.I know it sounds terrible, but he's also gained a lot of weight since we've been together. We've tried going running together, then we tried signing up for the gym and going together. It all lasted a while, but eventually he falls through. He stopped going running and he recently cancelled his gym membership. I don't know how to tell him that I want him to lose weight without really hurting his feelings. He's really self-conscious about his weight, but does nothing about it. This, along with what I've mentioned earlier, are things that I think are affecting my sexual attraction toward him.I'm attracted to him, he's very handsome, and I love him a lot, but I just haven't felt like having sex with him. I just don't feel turned on to him.A few nights ago I was out with a few friends of my boyfriend and I. We were having a good time and had a lot of fun together. We end up meeting up with one of his other friends who I've met like four times. So we're all dancing and drinking (maybe a little too much) and I notice his friend keeps looking at me. We make eye contact a few times, but I look away.So we're on the dance floor and I'm pretty drunk at this point, so is everybody else. Me being the loving drunk that I am, I put my arm around my boyfriend's back as well as his friend. We're just having a good time and I didn't mean anything by it. He got a little handsy with me, but I didn't think much of it. I'm kind of the same way when I'm drunk. It wasn't anything inappropriate.I forget exactly what happened happened to everybody else, but they were somewhere else and his friend says he has to go to the bathroom. On our way to the bathroom we stopped and we're talking about something. He tried to kiss me but I turned away. He immediately apologized to me and I told him not to worry about it. Nothing happened. I'm not going to lie, I was kind of turned on and I'm pretty sure he knew it. His friend is pretty attractive.So we end up at the next bar. We're dancing, drinking and just having a good time. At one point we were all together and I had my arm around my boyfriend and his friend again. I remember I was dancing and he starts smacking my ass. Still being turned on from earlier, I thought it was odd he was doing that because my boyfriend was like two feet away. Whatever. I turn around, look at him and turn back around.I don't remember if something was said or what provoked him to do this, but he gets my attention, pulls put his phone and shows me a nude. I was so turned on at that point, but I didn't do anything. I asked him if that was him (I was really drunk at this point, I don't even remember the picture). He said "yes" and I said "nice". At one point I remember him telling me that he doesn't think he's attractive, I countered and told him he was. I was just trying to be kind and speak the truth.We message each other on Snapchat every now and then and he's a good person. I don't know him as well as my boyfriend does, but that much I do know. I think he was just too drunk? But also alcohol brings out the truth.I'm not going to cheat on my boyfriend, but I don't want things to be awkward with his friend because I see him every now and then. I have no idea if this is a sore subject for his friend, but I want to make sure we're cool. He's been kinda short with me and I'm not sure if it's because he busy, or he's just embarrassed.What do you guys think of these situations?How can I tell my boyfriend I want him to lose weight? Do you think that my lack of sexual attraction is due to both my broken trust as well as the weight he's gained?Should I talk to his friend about what happened and ask if we're cool and kinda just set things straight and let him know that he's not going to get anywhere with me as long as I'm in a relationship?I really appreciate that you've read this for. It's kind of a long read and I also appreciate any advice!Thanks!