Warren Ellis has said that he thought Kitty was 18+ and depicted her as such, so she was an adult. The age gap was still significant, but she was certainly intended as legal age. So, no, the relationship certainly wasn't platonic and although Pete was, presumably, at least 10 years older than Kitty, it was a proper, grown-up, relationship for both of them, a real partnership, not a skeevy older man and his jailbait.