Hi,
Sadly, marriage records in Newfoundland very rarely give the parents names. It would sure be a lot easier if they did.

I'm not very familiar with the St. John's records, most of my research is in the Harbour Grace area. Looking at the 1794 census, two things stand out.
First, John is Roman Catholic. This makes me wonder if he is actually any relation to Henry Luffman/Loughman in Harbour Grace, since Henry is Anglican. By the time Henry married, there was toleration of Roman Catholics, and a functioning RC parish in Harbour Grace, so he would not have been forced to marry in the established church unless he converted of his own volition. Also, Roman Catholics in Newfoundland were almost always Irish. Are the Loughman's you are researching in Devon Catholic?
The other thing is that the census doesn't explicitly state that the male child is his. As this appears to be a household census, the male child could belong to one of the servants. I wonder is it possible to find out more about Mary Macdonald? Looking over the census, she seemed to have a lot of property and people looking after that property or renting from her.
John Loughman, boatkeeper of St. John's sounds to me like an Irish servant hired by Mary Macdonald to look after her boats and stores over the winter. Might be a different instance of the name than Henry of Hr. Grace.
Of course, John Loughman might still have married and left descendants. However I would expect he would have done so at the Roman Catholic Basilica. In those days, it was still a Cathedral and confusingly both the RC and Anglican Cathedrals were both in honour of St. John the Baptist. I can't find any on-line records for the Basilica. It's possible they are not in the provincial archives - their collection of RC records is less complete than other denominations. You could try contacting the diocese archives directly - they have an excellent archivist who may be able to help.
http://www.stjohnsarchdiocese.nf.ca/archives.aspI'll try to remember to take a look the next time I go to the Provincial Archives, but as I don't get there very often it might be a while.
Re the 1836 census, the only Lion’s Den I can find is on Fogo Island, a long way from St. John’s or Harbour Grace. Not to say that one of John or Henry’s descendants couldn’t have moved there. I had a quick look in the on line Fogo records, you could dig around there a bit more, you might turn something up. Sadly, the really old records for Fogo were probably at Twillingate and almost all the Anglican records were lost in a fire.
Jennifer