I'm looking for an all human fic where Spike and Buffy at first are in a sexual but not romantic relationship. Buffy is a widow who has decided she will not fall in love again because it's too painful. Spike is a bachelor who feels guilt over his best friend's death in an accident that occurred right after the friend found out about his wife, Drusilla, cheating on him with Spike. Spike has also sworn off love. He and Buffy live in different cities, I think, but they get together on weekends for their rendezvous. At some point they go off on a longer vacation together at which time they each realize they are starting to have feelings and so, not being aware of the other's feelings, they quickly back off and decide that it is over. I don't really recall what the circumstances are that bring them together again, but they do end up together. Spike might go by William, I don't remember.

I remember this one... I'll try to remember the name, but yeah, she's some high powered professional of some kind and he's some kind of billionaire businessman.

They might even live on opposite coasts, so they don't see each other often, but they go as each other's dates to professional functions, and have all sorts of things in common, really enjoy spending time together, and have crazy hot sex, but still think they're totally unattached and just great FWBs... Until that vacation, maybe on a Caribbean island (?).

Like I said, I'll keep thinking about it. Just thought I'd put in more details as I remember (hopefully they're close to accurate) to see if it helps jog someone else's memory.

Ok, I think I found it. At least, I found the one I was thinking of: The Arrangement by jamesgirl

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=22473

That's the one, thanks! I should have posted this a long time ago, it's been in the back of my mind for weeks!
I forgot that the cheating didn't actually occur, it just appeared so to Spike's friend. I felt I should make that clear so as not to turn anyone off of reading it.