http://www.fanfiction.net/s/3938720/1/Same_Coin

Sirius and Snape get detention together and discover that they love each other because they have similar pasts. Then James finds out that they're having sex and gets mad. Sirius proceeds to discover that Remus' werewolf side has taken James as a mate, even though James continues to date Lily. Sirius and Snape find out that they only fell in love because Remus cast a spell on them. Later on, the duo discovers that the spell was working, so their love is real. Oh, and Lily's a bitch because the author hates her. All this in ten chapters and several gratuitous sex scenes. Joy.