What constitutes cheating?

I'm married and so is the coworker/friend who I have been texting back and forth with for about three years now. At first, we just talked about work and other random things. At some point, not sure when, we started adding in all kinds of sexual innuendos. As of late, we don't even bother with those any more. Almost all of our texting conversations these days consist of us telling each other what we'd do to each other if we were allowed to do things to each other.

The thing that perplexes me is I'm not usually attracted to a smaller guy. I generally like to feel smaller than the guy, physically speaking. He's almost the same size as me, in fact, I think he only weighs ten pounds more than me.
I am overwhelmingly attracted to his personality and the way he acts towards me. He makes me laugh so hard; and so often when we're chatting at work about anything and everything, he says the thing I was just about to say and vice versa. He teases the heck out of me and giggles like a school girl about it. He notices and giggles at so many of the goofy things I do and say. The way he grins at me and calls me by the nickname he gave me every time he sees me at work makes me smile, and I just can't help it.

Is this cheating? HELP!