Archibald LEARMONTH {M} [P7] = Betsy CROY {F} [P426]

Married Not Married
Archibald:

Born 25 Nov 1777, Sanday
Christened 27 Nov 1777, Sanday
Died 28 Sep 1860, New Ortie, Sanday
His christening was witnessed by Archibald Learmonth Sen and George Drummond.

David {M} [P427]

Born 28 Dec 1808, Sanday
Christened 30 Dec 1808, Burness, Sanday
Died 3 Mar 1838, Lady, Sanday
Buried 6 Mar 1838, Lady, Sanday

In the Sanday OPR, David's records state he was the 'Natural Son' of Archibald Learmonth and Betsy Croy. This would indicate they were not married.
There are two Archibald Learmonth's who could have fathered the child.
Option 1
Archibald Learmonth P7 would have been 31 at the time of birth of the child and was living in Sanday, although married to Barbara Dearness.
Option 2
Archibald Learmonth P410 would have been 37 at the time of birth and staying in Kirkwall, being married to Christian Miller. He would probably have visited his parents in Sanday occasionally, but Archibald P7 would be the most likely candidate.