Jack is the only one he could talk to. Spike isn't comfortable with this sort of thing, at least not at the intensity they're thinking of and he can understand why. He's always felt at ease talking to Jack about sex because he always makes it feel so natural and easy. It's never been hard to open up to him right from the start.
"I wouldn't ask if I didn't feel confident." And after everything that has happened lately this level of trust feels incredibly important.