It's not that the X-Men won't kidnap Franklin. It's that the X-Men would be stupid to try.
Kidnapping a random mutant teen from Botswana is one thing. Kidnapping
the most powerful being on Earth who lives with the
Fantastic Four? A being whose
genius sister can get her
Uncle Doom to do almost anything for her? Even if the X-Men were to kidnap Frankie, they'd have one Hell of a time keeping him when the combined might of the FF,
Avengers, Inhumans, every other non-mutant superteam, and a few cosmic beings come knocking on Krakoa's front beach... not to mention, again, that
he's the most powerful being on Earth.