I'm looking for a story, it starts in Season 7. Buffy fights a demon that covers her in slime when she kills it. The slime makes her loopy and horny. She heads back home, where Giles and Anya come in contact with it and get horny too, and with each other and start doing it in the front yard. Only Spike is immune to it.
Buffy is all over Spike, and he is still racked with guilt and terrified of what might happen later, if he let's her have her way with him while under the influence of the slime.
Buffy pins him down, and let's him know. She has forgiven him but couldn't say it until now. The slime had worn off but she kept acting like she was to find out what Spike really thought about her and the bathroom incident. And they make love for the first time.
I, also recall, Buffy and Giles sent the Potentials away for the night, somewhere because they didn't want the girls to come in contact with it. And have to deal with, over 20 teenage girls, running around Sunnydale looking to get laid.
Also, it is revealed Giles has a piercing on him called either as Al, Alberta, maybe a Arthur in it? | 
