J11, you're quite right that Antenuptial Fornication was fornication before a marriage. But Isobel Gordon wasn't the other guilty involved - it was Janet Cruickshank, the mother of Helen Duff.
The full quote, dated 11 Dec 1837, is "Compeared Alexander Duff, Cooper, Muir of Rhynie, guilty of antenuptial fornication. He was rebuked, exhorted and absolved from church censure." The Banff Kirk Session had dealt with matters as recorded there on 22 Jun 1835. As I mentioned previously, on 25 Dec 1834 Alexander Duff had married Isobel Gordon, who of course was not named in the KS. Helen Duff was born in that summer of 1835, so was conceived in the autumn of 1834 the act thus being 'antenuptial'.
On the surface, it's taken some 2 and a half years for the Huntly KS to catch up. I have no explanation for this, other than that I've been told that the Kirk Session records for Huntly for 1826-1836 are missing. I've found no evidence of there being more one cooper in Huntly in 1834/35/36, never mind named Duff let alone named Alexander Duff; on the contrary, implicit in Janet Cruickshank accusing "Alexander Duff Cooper at present in Huntly" without qualification is that there was one and only one of that description, also that he wasn't always in Huntly is implied by the phrase 'at present'.
I'll probably never know what went on that summer of '35. Janet Cruickshank was "exhorted to reflect seriously upon her situation" (being her sixth-lapse in fornication *), and maybe she did, for by 1841 she was married to a John Hardie seven years her senior and they lived in Carmelite St in Banff without further progeny until their deaths in 1870/71 (by which time, Helen Duff had continued in her mother's footsteps). Maybe Alexander Duff scarpered in fright with Janet Cruickshank in hot pursuit seriousliy reflecting upon his position; maybe he was two-timing; maybe her accusation in Banff came as a terrible surprise. Maybe Isobel Gordon was steaming mad, maybe not. Who knows?!
* I know about two of her lapses, another three are possibles, the remaining one (and any others that escaped the KS's beady eyes) is entirely unknown.
I'm personally confident that I talking about one person; it'll need new hard evidence for me to reconsider; it's his parentage that is the puzzle. I did say he was probably born in Banffshire: true enough, but I feel he was born in Banff itself - being 'Duff country', being a cooper by trade perhaps down by the harbour, and where Janet Cruickshank had lived for some years, and where the father of her fourth lapse was a shipmaster named George Hutton (and I try not to read too much into that...!)