I don't think Buffy and Faith would be sexy. The idea of it makes me cringe (not because of girl-on-girl action, which is no problem, but because of the characters themselves - kind of like the idea of Cordelia and Connor almost, though not as bad, just the same kind of cringe).
I think Spike would be sexy with just about anyone, but not Xander. lol He and Faith would've been sexy if Faith could tone down her "I'm in charge, I use sex as a weapon" vibe that just about kills all her sexiness. If she could let someone else be at least equal to her in that arena, they would be very sexy together, IMO.