Xander once had a relationship with Giles (for about a summer) but Giles was an idiot and Xander ended up getting really hurt (comes out in backstory). Spike finds Xander working as a stripper: Xavier St. Martin & he's also a warlock (a really powerful one). They get together and then Wesley shows up (having been hurt by Angel) and fits in with them, then there's Oz and he and Wes get together. They end up going back to Sunnydale because Wes has a vision and Xander puts glamours on each of 'em...
I could go on but if I haven't recognized it from that I'm not entirely sure you will, lol! It's pretty good and I'd love to find it again because it's bugging me knowing I must have a link to it somewhere...
Thanks!
ETA: Not The Only One by Gypsy Gray - Thanks everyone!