This is making me nuts! It's a recent story, multiple parts, one of which ends with Spike walking in on Angel fondling Xander. Spike says "Aren't you two a treat"--or something similar. Then they go on to have a three way so that Spike and Xander can bond/mate/whatever.
If I remember correctly, the story starts with Xander pretending to be Spike's sub, then Xander asks to be Spike's all the time.
Help, please. Great thanks in advance.
And, my apologies to the author--I'm horrible with names, but I remember a good story!