Emotionally cheating vs. Physically

My boyfriend recently told me that he didn't physically cheat on me but he did emotionally cheat on me by signing up for dating sites and single web dating sites like flirting with other women, sexting, nude photos etc.... I am trying to figure out the perception behind this and why he couldn't do this with another women face to face vs. online. Of course, I do not condone this at all in any means but I know that there is a reasoning behind the actions people commit. Maybe I am thinking to deep into this.. I'm smart, young, intelligent and bright. I'm 23, and he's 33. We have a pretty solid relationship going and for the past 3 years things have been great and all of the sudden he tells me this and I am so emotionally wrecked because he doesn't at all seem like the type of man to cheat at all. He always seems happy around me, his family and he tells me he's happy but then this occurs. I feel like there might be a communication error maybe we need to communicate more. We have great sex and that's always been a bonus for us. My question is why do men cheat on good women? I just don't understand.

Thank you for your feedback.