He is absolutely in love, but cheated.

we had been dating for 5 months. He facetimed my mom for permission (yes, im old fashioned and still believe in that) he cried out of emotion and so did my mom. We have been friends for 2 years, we know each other so so well. He loved me so much, from the beginning of the relationship. I slowly caught up to him in feelings. I fell hard. I was so in love and so was he. We thought of our life together so much and felt so real, we thought about how many kids we were going to have and our beautiful home. He always had trust issues with me deep insecurities but I have never given him one good reason to not trust me. I'm so loyal and would never dream of hurting him. HE on the other hand has proven other wise, and yet I would always accept his apology and everything else he had to say and fall back into his arms. I am on vacation with my family, he couldn't come because of work. Im on vacation for a total of 1 month and a half. We texted everyday facetimed almost every night and were still in complete love. Still talking about kids and marriage. He had a dream that he was holding our baby, which I ranted to my mom about and she thought it was the most beautiful dream. 2 or so days after this dream I come to find out that he had been dirty messaging a girl he had previously hooked up with. more than once. She has a boyfriend too. When I say dirty I mean bottom of the sloppy joes over-boiled pot, dirty. I saw and read the whole conversation. She told him that they don't have to continue if he doesn't want to. To which he replied I do want to continue again and again. So they continued. This was all happening during the 3rd week that I was gone, and they had this conversation after his baby dream after all of this and during our lovey dovey texts. So she went on to ask about my where abouts and he said that I was gone and wont be back until mid august, that he is "solo dolo." They talked about where they would do it when they would do it, sent pictures along with the most unworldly of texts, mainly from his disgusting pig-brain. They never actually hooked up. Mainly because she could never get away from her boyfriend or was too busy, but he was feigning for it at every opportunity every minute every hour. During work breaks, 15s or 30s, in his car in her car anywhere and everywhere. The morning I found out his mom called and told me how much she loves me and how incredibly angry she is at her son. She too wanted us to get married and have babies. She loves me soooooo deeply. Along with the rest of his family, which I have spent a lot of time with both with him and without him and just the family. She cursed out her son, and want me to forgive him even though she knows how bad he fucked up and how much it hurts. SHe understood completely though, if I broke things off, but asked me to think about it. Which I did. and I it broke off. I'm having a hard time letting him go because I still love him and am deeply saddened by the fact that he would give into temptation so easily. I wish I could forget about this and love him and be okay. I know he deeply regrets it and still loves me with all of his heart. Why would he do this? please if any one can sense any kind of logic, please! help me understand his behavior. Help me see past this and ration with him.