Caught my boyfriend texting an old flame....

Not necessarily. Maybe you just aren't giving him what he wants in the relationship? I could be wrong. If he wants sex and there's a lack of sex, he wants to know that he's still sexy. And he's talking sexy to someone who gave him what he wanted. If that's the case, spice things up for him so he won't want to talk to other girls. Girls cheat for the same reason. They want what they're not getting, so they get it. He hasn't cheated on you, but it could get to that point. I'm not ruling out that your feelings are hurt. Yes, it's insulting that he wants to talk to other girls, but if you still love him then stop thinking about how you feel and take action. Make him want you and only you. If you come out arguing and fighting with him about it because your feelings are hurt then you might end up breaking up and he'll probably seek out that other girl.

Again, I could be totally wrong. I can't read his mind and I'm not in the situation. Just giving my take on it and a possible solution.

But no, he hasn't cheated on you. Cheating would be meeting the girl in person. He's just texted her, so make him stop texting her and get the relationship back on track.

There is no book of rules that says yes or no to that question. It is "emotionally cheating" if you feel it is. By "cheating" I mean "an action which calls, in your mind, his commitment into doubt, or which destroys the relationship of trust between you". I would feel cheated, especially (as you have noted) he tried to turn it back on me. It was his fingers that typed the sexually suggestive messages, not yours. It is your choice, but I would be dumping this dweeb. Do you want to wait till he screws her (your fault again, of course)?