Narrator-Ted said that she was a big part of his kids' lives, but he didn't say that she never had her own kids. I don't think the writers have closed that door yet (non-biologically I mean)

What I don't get is why everyone's assuming she'll never adopt. It's not exactly the same, but it's still real parenthood. Ted says she'll never be a pole vaulter; that doesn't have to be a euphemism. I never heard him actually confirm that she'll never be a mother after she does all her world traveling.