It happens once. When he's working late and everyone has gone home, except Spike who doesn't have one.
Spike may not be able to touch but hell, can he talk. His sinful voice and wicked images bring Wes to a crescendo that leaves Wes panting and dazed.
He figures that it will never happen again. A one time thing because things like this just don't strike more than once. But when Spike comes sauntering into his office after hours, Wesley amends the theory that maybe lightening can strike twice after all.
And a hell of a lot of times after that as well.