This has bothered me for the longest time. As someone who hates these kinds of inconsistencies, I feel your pain, Aragorn2.
Fact is, Lewis likely didn't realize when writing PC that Narnia began circa 1900. He didn't write most of MN until years later, so the possibility of a historical inconsistency didn't rise until the publication of later books.
Usually when I bring this up, people tell me that it was probably a 'wrinkle in time' type of thing. When the pirates passed through the portal to Narnia, they not only passed through space, they also moved through time up to some point after the creation of Narnia. I guess it's possible, but it's never satisfied me, since I don't think that's what Lewis had in mind. Whatever floats your boat, I guess (no pun intended,
).
I've always just had to square with the fact that there actually were pirates living from 1900 to 1940 (which there were), and that they were the ones who founded Telmar. Though they weren't the glorious pirates of the 16-1700's, there were pirates existing at the time. And pirates, well, pirate. In all eras, they've indulged themselves in theft, vandalism, and yes, rape.
Wow. That was a really long post...