On a completely clinical note: If you were having condomless sex with him on a regular basis, you are also most likely going through semen withdrawl, which intensifies the break-up feelings.
Out of left field, I know, but there you have it.
I dated this one guy, it was only for about 9 months and it was the most turbulent, tumultuous relationship I'd ever had. There were times were I never thought a relationship could be that much fun and then I was left staggering at how much of a f*cking @sshole he could be. We went overseas on a trip, had one of the best times of my life (I'm now at the point I can look back at that trip fondly again), he asked me to move in, discussed adding me on some key things...and then a week after we got back, he cheated on me with his bat-sh*t crazy ex (who showed up a 3am one morning, trying to break in to "talk").
I'd made the mistake a couple of times of listening when he said he missed me and realized just how big of a mistake he'd made in breaking things off. Even after the whole cheating thing, where I moved not only states but timezones to get away from him and the whole smoking wreckage, I was still so wrapped up in him and he was doing all the "right" things you see in the movies--sending me flowers every week, presents, etc.--that I went for it again. A third time. I don't often make the same mistake, but apparently when I do, I make sure it's a doozy.
Anyhow, the final time was when I finally realized that I couldnt' take the up and down (and knowing he probably was going to/had already cheated with Psycho again) and I made the clean break. I told him exactly what I thought about him in his driveway and then drove back to Colorado and I've never contacted, nor had contact, with him since. Despite his best efforts. Daily text and IM messages. Getting mutual friends to tell me just how "heartbroken" he was, what a changed man, blah blah blah.
I dreamed about him at least a couple times a week for nearly a year after that and he still occasionally makes appearances over three years later. Only person I've ever dated to do so. He was so totally the wrong guy and I feel that his chapter in my story was to show me things I may never have experienced on my own-- and to provide a foil to the amazing guy I'm dating now. I adore Ted like you wouldn't believe, but I know that some small part of it is that I have a perfect idea of just how great he is as a boyfriend, because I've been with the up and down type.