Friends who Fuck? WTF?

I am not unintelligent, and I am emotionally savvy enough to understand the dynamics of most relationships, but the one phenomenon I don't understand (and possibly have some cathartic resolution to deal with) is that of friends who fuck. I'm not talking about the "I'm heartbroken, you're drunk, we're good friends and fell into bed together and now will feel awkward for a little bit." That I understand. I'm talking full-on, can't keep your hands off each other, looking for any excuse to be alone together, years on end, really good friends fucking.
Not to be confused with the following scenarios, which I also understand:
-Really, really good friends who really like each other but are not sexually attracted to each other.
-Being really into having sex with someone you don't really like (it's not nice, but it happens.)
-"normal" relationships in which you really like someone and really like having sex with them. They are called "Boyfriends" and "Girlfriends."
-Starting out as really good friends but, being sexually attracted to them (scenario A plus sexual tension), thus becoming "boyfriends" and "girlfriends." (scenario C)

But, here's where I get lost (and, of course, tell a story). Starting out as really good friends who really like each other, feeling mounting sexual tension build between you, giving in to it, discovering that you really like having sex with your really good friend, and then swerving off the road to become Friends who Fuck.

So, The Anti-hero and I met because he was dating my roommate. We didn't really become friends until she dumped him, but then we very quickly became very good friends. We even made up a myth about being long-lost brother and sister. We decided when both of our leases were up, we would be roommates.

Then we kissed. And decided we would never do so again.

Yeah, right.

We continued on our merry way, being really, really good friends, and looking for apartments. Another girl was going to live with us too, she turned out to be The Crazy (remember her, she'll be back).

So, after being really good friends and roommates for about 6 weeks, the tension between us was palpable. You could cut the air between us with a knife. Even The Crazy noticed. And forbade us to sleep together. Big mistake.
Here's the deal. You want Sugar Twat to do something? Tell her not to.

So we did.

And we liked it.

A lot.

Plus, because of The Crazy, it was a secret. Let's face it, secrets like that are sexy. We started doing just about anything to be alone together. I got The Crazy a job where I worked so that we would have guaranteed Sundays together until 1:00 pm. We would go to Stop and Shop together and park behind the store. Once we even went to where he worked in the middle of the night (not uncoincidentally, a furniture store. Okay, I'll admit it. That was pretty hot).
But, something wasn't right. We were really really good friends, and we really really really liked having sex with each other. But, he didn't feel "That way" about me. Honestly, which part of the equation was missing?
There was a lot of strife over this, but still the fucking.
The Crazy moved out. Wonder Muff moved in.
After about a year, he moved out. We thought that this would solve the problem.

It didn't. Now The Anti-hero was living alone. Less sneaking. He had no roommates to hide our secret from.

More tears, more declarations that we would stop.
We didn't. We couldn't. It was as if we were addicted to each other.

Wonder Muff and I moved to a different apartment. Now instead of around the corner from each other, I lived across town from The Anti-Hero. Didn't matter. He would just drive over.

I moved into my own place. Now we were both living alone. I could have him over for dinner first. More fucking, more tears.

Surprisingly, this story does have a happy ending. Finally, after four years, we realized that our sexual appetites for each other would destroy our friendship. I met a guy and moved to Boston. He met a girl and moved to New York. We remained close. We visited each other. Six years ago, when my heart was broken more terribly than ever before, he took me to the Vineyard and helped mend my heart with the innocent, platonic love we now have for each other. He has turned from being the man who breaks my heart to being the one who heals it. I now consider him one of my best friends, with no strings or disclaimers. Or sex.

But, there is still that nagging doubt in my head. Why didn't he want to be with me in a meaningful way? We had an emotional bond. Clearly we were attracted to each other. Our "non-relationship" lasted for 4 years, longer than all but one of my "real" ones. Was it just that he didn't want a commitment? God, we were young! When It started I was 20, he was 23. Or was it me? Was I not "girlfriend material?" Am I now? I do know this: I did learn that one should never settle for less than what one wants in a relationship. And that I never again allowed myself to be used, even by someone I loved.