I need a MAN'S opinion please. Someone in their late 20s.....?

I've been dating my boyfriend for about 7 months now. Things are going great between us. We've talked about moving in together. We're planning a vacation this summer. I need my space just as much as he needs his, so I don't obsess over him whenever we're not together. I'm not the jealous girlfriend type, and I'm ok with sitting at his place on a Friday night while he's out with his friends. Our sex life is great. We both agreed that it's very healthy. He's very sweet to me. He goes out of his way to make me happy sometimes, and I do the same for him. He tells me that he's very happy with the way things are going between us.

But I can't help but feel that sometimes I'm not enough for him. Sometimes when he's sitting next to me texting or on his Facebook, I notice that there's conversations between him and his ex's in there. I never read them or ask about them, because I trust him. But he has told me nothing but bad things about them. So why is he still talking to them? Recently, I got a Facebook message from one of his ex's (pretending to be someone else) telling me that he's been talking to her behind my back for months, and he told her that him and I aren't dating. I asked him about it and he was pissed that she would try to ruin things between us. Then he told me a few days later that he's known for months that she's wanted to get back together with him, and she texts him late at night sometimes. I guess my question is, why is it when a guy finds a great girl who isn't jealous, obsessive, clingy, slutty, and just crazy about him, that he still wants attention from other girls? Every guy I've dated has cheated on me, and trust me, knowing that you're not the only one is a shitty feeling. I feel like I'm doing something wrong here. Is this normal behavior from every guy and I just need to deal with it? Or do I need to talk to him about it? I just feel that I deserve to feel the same way I'm trying to make him feel. Like he's the only one for me.