AH Joyce is Spike's maid and Buffy comes to visit. They ride horses which leads to to other things

Just like the title says. Spike is a wealthy man and Joyce works in his home. Buffy comes to visit and Spike takes her out on horseback where they tease each other and end up getting dirty in a barn

His Dirty Little Secret by jamesgirl

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=24000&warning=NC-17

Summary: William 'Spike' Giles and Buffy Summers are both students at Sunnydale High, but they lead two very different lives. Spike has it all – looks, wealth and popularity. Buffy has nothing – since her parents divorced, she's been living in the servants' quarters of the Giles mansion, where her mother earns a meager living as a maid. But beyond these differences, they have a secret relationship – at school, Spike pretends not to know her, but at night he visits her room with growing regularity...

no that not it.

It's not His Dirty Little Secret. but thanks anyway

Break Me Gently is it. Thank You very much!