An unaware Buffy ends up at a swinger party, she's paired up with Spike who's not really a swinger.

Tthere is a fic that has a party where all of the couples are swingers but Buffy does not know this. When she ends up with Spike, Harmony states that she feels sorry for her. Spike never sleeps with the women that are chosen for him but ends up paying them to be quiet so his wife does not know. Hope this is enough info, if not just let me know.

The Midnight Society by by foreveryoung

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=33892

Summary: After five years of marriage Buffy finally decided to officially changing her last name from Summers to O'Connor. Then, a few weeks before the couples 5th anniversary, they attend a party that causes much confusion and emotional stress for the petite blonde—putting her faith in her marriage and her faith in love on the rocks. The man she spent 9 years with becomes a stranger in one single night. However, at the party she does meet William...

This is a WIP that is a particular favorite of mine so I recognized it right away. I long for updates...

Tank you Thank you. This was it!!!!!!!!!! I can't believe that its a WIP.

Thank you Thank you. This was it!!!!!!!!!! I can't believe that its a WIP.