I just did some digging, and apparently The Stolen Earth/Journey's End is supposed to take place in June 2009, though I'm not sure the source on that. Regardless, for the sake of discussion, we'll stick with that date, and Journey's End would be the first time the two ever meet each other, and as pointed out, at the time Martha is engaged to Tom Milligan from the S3 finale. Children of Earth takes place in September 2009 according to the date stamped on various computer screens, and Martha is said to be on her honeymoon. So, in the span of three months, Martha met Mickey, broke up with Milligan, and married Mickey. About the only thing I can figure they have in common is they're both resentful of Rose Tyler. Now, I'm hardly an expert in these matters, but I'm pretty sure resenting the same person is not sufficient grounds for two people to get married.