The fascinating story of a thirteenth-century noblewoman who left her husband and moved in with her lover, with, amazingly enough, her husband's blessing.
Margaret Gatesden (or Gatesdene or Gattisden, etc) was born sometime in the early or mid-1250s as the daughter of Sir John Gatesden, a landowner in Sussex and Surrey, and Hawise née Courtenay, widow of John Neville. Margaret had older half-brothers from her mother's first marriage, but was her father's only surviving child and heir, and inherited manors in Hertfordshire, Bedfordshire and Sussex. She married Sir John Camoys, who was born around 1247/52: in November 1276 he was said to be 27 years old, and in April/May 1277, he was either 25, 26 or 30. John inherited lands in Surrey, Norfolk, Northamptonshire and Cambridgeshire.  Margaret Gatesden and John Camoys' son Ralph Camoys was born in or before November 1273: he paid homage to Edward I for a manor in November 1294, and must have been at least 21 then.  Ralph Camoys was a long-term adherent of Hugh Despenser the Elder, and in 1316 married Despenser's youngest daughter Elizabeth as his (decades-younger) second wife. Ralph's grandson Thomas Camoys was born around 1350 and lived until 1421, having commanded part of the English army at the battle of Agincourt in 1415. Thomas's daughter Alice Camoys, Margaret Gatesden's great-great-granddaughter, was the mother of Edward IV's friend William, Lord Hastings (d. 1483).
Sometime in or before 1277, according to the Complete Peerage, Margaret Camoys née Gatesden fell in love with another man, Sir William Paynel, lord of Trotton in Sussex, who many years later inherited lands in Wiltshire, Hampshire, Sussex and Surrey from his older brother Thomas Paynel (d. 1314). William was said to be 60 years old in 1314, so was born c. 1254 or rather earlier, and was therefore apparently around Margaret's own age.  Margaret left John Camoys and went to live with William. Rather remarkably, John accepted the situation and responded to the situation with astonishing kindness and benevolence. He stated on 11 June 1285 that 'I will and grant … that the aforesaid Margaret is to live and remain with the aforesaid Sir William', transferred his rights to the greater part of Margaret’s inheritance to Paynel, and gave up his claims to her goods and chattels. In 1300, it was said that Margaret 'lived with the same William … with the consent and by the will of the said John, then the husband of the same Margaret'. On 9 June 1281, there's a record of a '[f]eoffment by John de Cammays [Camoys] to John de Kirkeby of the manor and advowson of Cotherstok and the mills of Pireho with their suits, all of which Sir William Paynel, who enfeoffed the said John de Kirkeby thereof, had by his gift and feoffment.' 
After Sir John Camoys died, Margaret married Sir William Paynel, by then her lover for more than twenty years. Edward I’s government appeared far more upset about Margaret’s behaviour than her first husband did, claiming after John died that she had no right to dower as his widow because she had 'abandoned' him, was 'guilty of the crime of adultery', and 'is living in adultery rather than in any other proper or lawful manner'. Gilbert St Leofard, bishop of Chichester, however, acknowledged that in early 1296 Margaret had 'solemnly and canonically purged herself' of adultery before Gilbert’s dean and treasurer, the prioress of Easebourne, four ladies named as Margaret Martel, Isabel de Montfort, Hawise de Houtot and A. Corbet, and 'many other married women and young maidens of the neighbourhood'. The bishop therefore declared her innocent of the crime and requested that she might be restored to her good name. William Paynel had also legally purged himself of adultery before John Pecham, archbishop of Canterbury, in 1288. The reaction of Edward I's government to the Margaret Gatesden/William Paynel situation mentions a recent statute 'concerning women leaving their husbands and living with their adulterers and not reconciled freely and without ecclesiastical coercion before the deaths of their husbands … in which it is expressly contained that, if a wife freely leaves her husband and goes to live with her adulterer, she is to lose in perpetuity her action for claiming her dower which might belong to her'. 
The awesome John Camoys, who evidently cared more about his wife's happiness than he did about his own reputation, died shortly before 4 June 1298, and Margaret married William Paynel in or before 1300.  They had no children during their long relationship, or at least, no children who survived. Margaret Gatesden Camoys Paynel was still alive on 16 June 1310 when she appears in an entry on the Close Roll, and died shortly before 4 January 1311, when Edward II ordered his escheator to take into his own hands the lands 'late of Margaret late the wife of John de Camoys, deceased'.  Her heir was her son Ralph Camoys.
Margaret's widower William Paynel died on 1 April 1317, leaving his younger brother John, said in William's IPM to be anywhere between 40 and more than 60 years old (!!), as his heir. It seems more likely that John was much closer to 60 than to 40, and that the two younger Paynel brothers were born around 1254 and 1257. William was said on 7 June 1314 to be too physically weak to be able to travel to Edward II and perform homage for the lands he had inherited from his older brother Thomas, and by 14 June 1316, he was blind ('deprived of his sight').  He had, however, married a second wife, Eve Dawtry, before 6 November 1314. She was the granddaughter and heir of William Dawtry and the widow of Roger Shelvestrode, with whom she had a son named John.  Eve married her third husband Sir Edward St John just a few weeks after William Paynel's death, sometime before 26 May 1317 when the king seized their lands because they had married without a royal licence. In February 1321, Edward II pardoned Eve, Edward St John and twenty men for supposedly abducting Eve from Cowdray in Sussex, "she being willing and assenting thereto". Edward's father John St John had acknowledged a debt of 72 marks to William Paynel in 1281, so the families had known each other for a long time. Many years William Paynel's junior, Eve did not die until August 1354, having outlived her eldest son John Shelvestrode. Her heir to her Paynel dower lands was William's niece Maud, daughter of his younger brother John; her primary heir was her grandson Roger Shelvestrode (b. 1334); and she had two sons with Edward St John as well, named John and Edward St John.
William Paynel's stepson Sir Ralph Camoys died shortly before 17 September 1335, when his lands were taken into the king's hands. In June 1334, Ralph's eldest son Thomas Camoys (d. 1372), grandson of John Camoys and Margaret Gatesden, was one of the two godfathers of Roger Shelvestrode, grandson and heir of Eve Dawtry, second wife of Margaret Gadesden's widower, while Eve herself was her grandson's godmother.  This is a rather fascinating illustration of how the Camoys, Paynel and Dawtry/St John/Shelvestrode families remained close, decades after Margaret Gatesden left her husband John Camoys with his blessing to live with her lover William Paynel.
1) CIPM 1272-91, nos. 178, 212.
2) CFR 1272-1307, p. 349.
3) CIPM 1317-27, no. 456; Complete Peerage, vol. 10, pp. 319-31.
4) The Parliament Rolls of Medieval England: Original Documents, Edward I Parliaments, Roll 11; TNA E 40/9069.
5) Parliament Rolls.
6) CFR 1272-1307, pp. 76-8, 349, 400; Parliament Rolls.
7) CCR 1307-13, p. 217; CFR 1307-19, pp. 77, 81.
8) CIPM 1317-27, no. 46; CFR 1307-19, pp. 198, 321; CPR 1313-17, p. 472.
9) Complete Peerage, vol. 10, p. 330.
10) CFR 1307-19, p. 328; CPR 1317-21, pp. 559-60; CCR 1279-88, p. 130; CIPM 1352-60, nos. 189, 271; CFR 1327-37, p. 459.