Spike is married to cecilia who is buffy's cousin. Buffy loves spike and when cecilia goes away on a trip makes her move. Spike ends up returning her feeelings and he leaves cecilia and marries buffy

That sounds like Do You Wanna Touch Me by PaganBaby...It's on Spuffy Realm

Here's the link..spikeluver.com/SpuffyRealm/viewstory.php

Hope that's it...:)