This song seems to have originated in a broadside, though it has clearly travelled since.

I did a search of the Bodleian Library Broadside site and found 13 hits. Most of them, including the earliest contained the version where the planter married the girl and she gave the convicts good usage on Van Dieman's land but there were a couple that contained the other sequence with the girl being transported for prostitution and the ship's captain marrying her, so both versions are valid.

Clearly, one interpretation of "good usage" implies sexual favours. However it is also possible that it simply implies acting in a kindly manner towards the convicts; trying to ease the misery of their existence in some way. Which it is I don't know, but while Lighter is right about 19th century sentimentality - though I think this became more prevalent later in the century, there is enough sexual insinuation in folk song for that interpretation to be equally reasonable.

Whatever your interpretation, I don't see any of the versions of the song tending to the giggly and leery. Transportation was no joke and the reaction of the landowners to poaching has always struck me as disproportionate. Most poaching seems to have involved villagers trying to eke out a diet that was otherwise insufficient to feed a large family. If you are desperate and unable to put enough food on the table for your family, you will take risks you would otherwise not have considered. Rural poverty was a very real problem in the early 19th century and the methods used by the authorities to deal with it - The Workhouse, transportation or hanging for relatively trivial offenses etc. were brutal to say the least.