Cheating....

What exactly constitutes cheating? I ask because Tall Guy and I are dating now- but Stupid Ex and I are sleeping together, too. I know this is wrong, believe me. But there's this incredible chemistry between us that just isn't going away. The sex is good, really, really good. And I know I should feel terribly guilty, but I don't really. TG and I are dating but he's not my boyfriend, and I'm not his girlfriend. And SE is single again, although we did have sex while he was still seeing What's-her-face. But I am thoroughly confused. Things are definitely slowing down with TG, and he specifically asked me not to sleep with my ex again. I guess that's the only thing I feel pretty bad about. We'll end up seeing what happens- it isn't fair to do this to him though, not after what his now-ex-wife did to him.