Well, he'd got that insinuation too, but because he doesn't generally feel ashamed of wanking (any more than he does about any other kind of sex) the insult unfortunately doesn't have the same impact it might on someone else.
Especially when banter and goading is their usual form of communication anyway.
And competitiveness, of course, but he likes the form Spike's competitiveness is taking right now. He grins widely in response. "Well, of course," he agrees airily. "This was just the beginning!" His grin turns wicked. "All the things I want to do to you, Spike. All the things I want you to do to me!"