Not sure if this is 100% what your looking for but in my fic Beautiful mistakes they are both human and Buffy is in love with him, he doesn't hate her but he isn't kind either when they get into a fight, he falls for her and get possessive and jealous and it is very much so NC-17 lol I hope that helps
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=33815&warning=NC-17Runner Up at the Cradle Of Humanity Awards Round 3 for Best WIP........................ Buffy and Spike have been best friends for as long as they both can remember. After one passion filled night together Buffy and Spike find themselves in uncharted waters. Cheating partners, a bottle of Jack Daniels, tight shirts, jeans and entangled limbs. Was the night all one big mistake? Will Spike realise Buffy's true feelings before he looses her forever? or will it all just be to little to late?
There is also Keeping secrets that sould kinda like what your looking for
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=35414Armed with fake I.D's 16 year old Elizabeth Buffy Summers and her best friend Cordeila Giles head out to L.A's hottest club intending on meeting up with Cordy's older, secret boyfriend Xander but when Cordy and Xander bail on Buffy she meets the man of her dreams Spike.
He is everything she has ever wanted in a guy fun, smart, sexy, and a wonderful lover only problem is he is 22 year old collage student and thinks she is the same not a 16 year old high school student.
How will Spike react when he discovers the truth? will he be able to forgive her? will he be able to put her age aside? and is her age the only secret she is keeping from him?