Is my boyfriend cheating?

Okay, we have been dating for a year now. About a month ago I went through his phone (without him knowing) and found flirty texts between him and a girl (whom he met while at a leadership seminar) from when we were dating for six months. We were having an off time during the relationship. There were only 20 texts or so and nothing intensely explicit or sexual, and he was the one to cut it off (she texted him several more times but he never answered). When i found them I confronted him and he admitted it, crying and apologizing. He said it was because he thought there was nothing left for us but talking to her made him realize how much he loved me. (Also, I looked through every single one of his texts and there was nothing besides this girl.) Recently he went on another seminar (HOBY), which enforces community, caring for others and many claim to make lifelong friends there. While there, he met one girl in particular (she was in his group) and got her number. He talks to her every day for a few hours, in texts and sometimes on the phone. I only know this because he told me about her, saying we were similar and would get along great. I (I think understandably) got nervous and possibly overreacted about him talking to her so much. He says her boyfriend abuses her and he enjoys being there for her because it makes him feel good to help someone and he says he is the only person she can talk to. I asked to see their conversations and he obliged, but he wouldn't let me scroll through the texts himself. When I asked him why he said it was because he didn't want to violate her trust and let me see the things she told him (about her relationship problems, etc.). Besides this, he is and always has been pretty protective of his phone and facebook password (one time he forgot to log off so I read his messages but there was nothing). He is a naturally defensive person, to me, his family, friends, teachers, about anything, from cleaning his room to me accusing him of cheating. He is also extremely friendly, but it is just good-natured. He tells me about his female friends and encourages me to befriend them, texts/calls/hangs out with me as often as possible, and appears to be genuinely happy and satisfied in our relationship. So what I am asking is: why is he so incredibly protective of his phone? he tells me who he is texting, even lets me read texts over his shoulder, takes his calls openly, but won't leave his phone alone with me. Is it just a privacy thing or is he hiding something? (Side note: one time his friend forwarded him a nude picture of a girl and he even showed it to me, laughing at her naivety for sending pictures.)