They all do it - it's part of the rock-star-entourage benefit package - getting laid a lot. Spike likes to get laid. Sex is vital and fun and a great way to blow off steam when he can't go out and kill something. Truth is, he could go out and kill something - six months now with the ability - but he doesn't want to see that conflicted, white-hat-with-a-moral-dilemma look on Xander's face. Doesn't want to see disappointment there, either.
So, they all do it. The bodyguards do it, well, all but Jack. Jack is...different. He never seems to be interested in anything but his job. Spike finds him a little cold, but the guy can kick ass and doesn't balk at the other aspects of the job, so if he wants to keep company with his hand, it's all the same to Spike. Carl likes tall women, usually those that are light mocha compared to his dark skin. Isha is kind of shy and sometimes winds up with the quieter, plainer friends of the girls who launch themselves at Spike. Ace likes boys, and if Spike notices that he trends toward tall, dark, boyish guys, well, that's not his to discuss.
Even Annie does it. She's discreet and particular, but Spike's seen her sending a few men and fewer women away from her hotel room door in the early hours. He just thinks "good for her" and lets her sleep late, if she will. Xander didn't do it for a long time. For a couple of years, his eyes followed petite blonde girls, but he always turned them down gently and sweetly, claiming that the manager's work was never done. After a while, he surprised Spike by going for a couple of guys, usually slim, compact men, nicely muscled.
Spike does it all the time, and he's not particular. He's got a finely honed radar that picks the most likely body out of the crowd and zeroes in, and he never, ever loses. He's picked girls with husbands or boyfriends, and, with nimble grace and nimbler fingers, managed to get them both into his bed, if that's what he wanted. Sometimes he goes for demons or other vampires, when he's feeling like revisiting the good old days. He'll admit that he's sought out big, brooding darkness and slim, ethereal craziness from time to time.
Groupies are the easiest; they're like takeaway or room service - delivered right to the door. Some of them follow him around the country, a few around the world. He rewards that sort of loyalty, if she's pretty enough or he's drunk enough. He's done some crazy shit in his time, but the craziest yet is going on in his head. Xander has walked in on a couple of scenes. It's not too unusual, because Spike isn't all that careful about closing doors or moving to less public spaces when the urge strikes him. The crazy part is the shuttered look he thinks he's seen on Xander's face, and the lurch in his unbeating heart when he thinks about it.
Xander's his friend. It's a word Spike doesn't use lightly. He had a friend, back when he was alive. A good one, a true one. He thinks Xander might just be the second person in his life who deserves to use that title. And you think about your friends in a certain way, and not in others. And that's just the way things are.