Can I just ask - are you getting the full names of the women from their children's birth cerificates? If so, there is always the possibility that they weren't actually married. That was (and is) by no means uncommon, and it's quite usual for the parents to claim to be married on censuses and their offspring's birth certificates.
Did you know, by the way, that 1817 Directory for Hull shows a John Grayburn, sailmaker, at 4 Posterngate (he's also there in 1803), and a Thomas Grayburn, agent to the garrison (no address)?