This is a very nice little story, but I have to agree that this is begging to be a longer story. I must say that using a shortage of males (apparently from the war) as a reason for polygamy seems a more realistic reason for a Harry/many story than most.
While a Harry/many story might have people expecting a fair amount of smut, its not really necessary story. I think this has been set up in such a way that the really interesting part of the story is the 'through the looking glass' quality of Rebecca and Rose learning about a new world.
If necessary I would offer to help you by acting as a sounding board for ideas, or possibly even a beta depending upon your needs.
Having said all of that, I can't help but mention that I am looking forward to the next chapter of Motorcycle.