it all started with buffy calling angel to tell them she and spike were coming to la (she wants to let angel down easy about her and spike little does she know angel has gotten a permanent soul and is now with cordy...i don't think he would be as surprised as buffy cause he has always known that she has been with other guys and i guess even though it is gross her and riley were kinda serious, but i think buffy would be devastated because a part of her would think that angel would always love only her and let's face it she has never been bothered about rubbing her other relationships in angel's face and while he never really fought for them to be together ie leaving sunnydale and iwry
anyway she decides to tell angel she is with spike now and angel says it is ok cause he has something to tell her (think his biggest problem with it is that it is spike)
anyway she would come up with the double date idea to show it doesn't bother her and also because part of her just doesn't believe it would be possible for angel to want to be with cordy and that she was no longer his #1 girl
cordy and spike would go along with it cause they would both want to make sure that they were,t second choice and angel would be forced to go along with it coz in his defense angel hates doing stuff like that anyway