Have had a few over the years, and come to the conclusion that I am not very good at remaining unattached.
Sex, normally great. Don't regret it, but have learned enough about myself to know it's not something I can do.
Spent a whole summer having ridiculously great sex and being mates with a guy. We hung out a lot, went camping together, met some of each other's friends. . .but it had to stop when I wanted it to be more a 'relationship' than he did. FBs is what we'd agreed, so the sex part stopped. Years and years later, he was moaning to me (online) about his lack of love life and actually said 'why can't I find someone like you?'....and I said 'Um...you DID'. He was quite embarassed.
The last FB I had happened just as I decided to split up from my asexual husband. Technically still married, FB said he was too busy for anything serious but didn't mind being the 'bit on the side' while I figured out what the hell to do about the marriage. It was really just shagging for about a month, then he suggested we go away for a weekend (for some shagging out of the earshot of his flatmates). I figured out during the trip that not only did I really like the shagging, I actually liked spending time with him, found him quite funny and interesting.
I did NOT want to get into another situation like with the other guy. So, when we got back, I mustered up everything I had and made a little speech about how this had been really great, but I was getting too emotional about it to continue on a FB basis, and since I knew that's not what he wanted, I was going to stop it right now. I even told him about previous FB and how I was starting to figure out having a FB wasn't for me. And I walked out the door.
So....two years later, we're married and expecting a baby.
I guess the only advice I'd give is that you have to know yourself. If you aren't able to remain unattached, you have to be prepared to walk away. Sometimes he'll follow you, but more likely he won't.