Having Spike suck his blood wasn't nearly as sensual experience as he had thought (not that he WANTED it to be sensual, and NOT that he had actually THOUGHT about it, no, not at ALL.) Having heard from Buffy and Anya about human ghouls, Vampire-groupies who let their vampires do these things on a regular basis, Xander expected SOMETHING more interesting than feeling Spike's mouth give him a slow, leisurely, VERY ineffective hickey.
Spike's HANDS, however, were a whole different story. As Xander lay on his back with the Vampire's naked, damaged (but still, naked!) body laying on top of his bare chest, the Vampire's cool lips attached to a wound he (well, they) had made just over Xander's right nipple, well, you'd THINK that Xander might be distracted by something OTHER than Spike's HANDS...but there it was. Cool hands on Xander's arms, on his back, almost lifting him up to drink from him as if from a holy grail ... fingers sometimes creeping up to the wound and caressing his chest, or sneaking up to brush his face or cup his cheek, then back to clutch his arms and draw him closer.
What do to with XANDER'S hands, however, was a big problem. He didn't want to be touching Spike's slim back or his muscled shoulders or (god forbid) holding the blond's head, while all the time murmuring....excuse me, NOT murmuring, SAYING VERY FRANKLY, "yes, Spike, yes."
Because he had to agree to the feeding.
More than once.
Otherwise (Spike said) the chip would engage and cause him terrible pain, thus Xander couldn't JUST say "I, Alexander Harris, 22 years of age and lifetime resident of Sunnydale California also known as Weirdworld, give you, William the Bloody, permission to suck my blood" only once. He had to (Spike insisted!) keep saying it over and over.