Mungo and Katharine are my GGGG grandparents, but I've never managed to establish their origins with any certainty.
A Mungo FINLAYSON was baptised in St Cuthbert's, Edinburgh 14 May 1742, which would have made him 32 at the time of marriage, but is it, indeed, our Mungo?
The only remotely likely Katharine / Katherine Millar baptism I've found is 1733 so she doesn't fit but she could be this Katherine's mother.
I would assume that the they are all related as Haddington isn't very big even now and the fact that Mungo was a witness makes it even more likely.
More research needed - if more information is available.