Based on published disease prevalence data and number of married couples according to the latest census, researchers were able to calculate expected incidence, and conclude that the number is greater than expected by chance.
 
First two cases were reported from the U.K.:
CROHN'S DISEASE IN A HUSBAND AND WIFE.
Whorwell PJ, Eade OE, Hossenbocus A, Bamforth J. Lancet 1978 Jul 22;2(8082):186-7
CROHN'S DISEASE IN HUSBAND AND WIFE [LETTER]
Whorwell PJ; Hodges JR; Bamforth J; Wright R;  Lancet, 1981 Feb 7, 1:8215, 334

two more followed in 1985:
CROHN’S DISEASE IN TWO MARRIED COUPLES.
Rhodes JM, Marshall T, Hamer JD, Allan RN; Gut 1985 Oct;26(10):1086-1087

Only two examples of married couples where both partners have developed Crohn’s disease have been reported previously from the United Kingdom. We describe two further examples; one where the index patient had developed Crohn’s disease before marriage, and the spouse subsequently developed Crohn’s disease and a second example in which both spouses developed Crohn’s disease after marriage.
 
If a prevalence of Crohn's disease of 35/100 000 is assumed then the chance of Crohn's disease occuring in an unrelated spouse is (35/100 000) squared - that is , about 1 in 8 million. As there are just over 12 million married couples in England and Wales (1981 census) only 1.5 couples with Crohn's disease would be expected to have occured by chance in England and Wales. The chances of finding four or more married couples with the disease as predicted from a Poisson distribution would be p=0.066 and the indentification of one further example would reach statistical significance (p for five or more couples = 0.019). Mayberry and his colleagues have reported a prevalence of 55.7/100 000 in Cardiff. If this prevalence rate is used then the expectation of 3.72 spouses developing Crohn's disease by chance is close to the reported total of four examples.

Unless further examples remain unreported, however, Crohn's disease seems to occur about as frequently in the spouse of a patient as would be expected by chance.
 

unfortunately, further examples were soon reported:
CROHN'S DISEASE IN MARRIED COUPLES [LETTER]
Holmes GK; Painter NS;  Gut, 1986 Mar, 27:3, 350

CROHN'S DISEASE IN MARRIED COUPLES [LETTER]
Lobo AJ; Foster PN; Sobala GM; Axon AT;  Lancet, 1988 Mar 26, 1:8587, 704-5
 
We describe a couple in whom Crohn's disease developed after marriage, bringing the total number of affected couples reported in England and Wales to six.

Using the figure of 35 per 100 000 for the prevalence of Crohn's disease in the UK, Rhodes et al. calculated that the chance of finding five or more married couples with the disease is 0.019; the chance of finding six or more is 0.004. However, these probability values are influenced by small variations in the estimate of prevalence.

The occurence of Crohn's disease in a greater number of married couples than expected by chance lends support to the view that an environmental factor, infectious or otherwise, is involved in the aetiology of Crohn's disease.
 

FREQUENCY OF INFLAMMATORY BOWEL DISEASE IN OFFSPRING OF COUPLES BOTH PRESENTING WITH INFLAMMATORY BOWEL DISEASE.
Bennett RA; Rubin PH; Present DH, Department of Medicine, Mount Sinai School of Medicine, New York, New York. Gastroenterology, 1991 Jun, 100:6, 1638-43

The development of IBD after marriage in the second spouse of 14 couples suggests either a chance or an environmental factor in the pathogenesis of disease in this patient population. In the United States, the prevalence of CD and UC is usually cited in the literature as 40 of 100,000 and 79.9 of 100,000 of the population respectively. Assuming that the population base of our practice is approximately 6 million, the chance that 2 randomly paired individuals will develop either disease is then 120 of 100,000 or 1.44 of 1 million, making it unlikely that 14 couples with IBD from this population would be married by chance.... Although the initial spouse had CD in 6 of 14 patients, the second spouse developed CD in 10 of 14... We found a high rate of IBD (36%) in the offspring of these couples. Although there was no apparent proclivity or concordance for one or another particular form of IBD among these 19 married couples with IBD, 9 of 12 affected children (75%) have CD. Moreover, when both parents had CD, all 4 affected children developed CD.

NOTE: The prevalence of CD in the second spouse (10 of 14 IBD patients) and their children (9 of 12) is in sharp discordance with CD / IBD patient ratio in general population (40 of 119.9), further supporting that this did not occur by chance.
 

INFLAMMATORY BOWEL DISEASE IN MARRIED COUPLES: 10 CASES IN NORD PAS DE CALAIS REGION OF FRANCE AND LIÈGE COUNTY OF BELGIUM
Comes MC; Gower Rousseau C; Colombel JF; Belaïche J; Van Kruiningen HJ; Nuttens MC;
Cortot A;  Registre des Maladies Inflammatoires du Tube Digestif du Nord Ouest de la France
CHRU Lille.  Gut, 1994 Sep, 35:9, 1316-8

Ten pairs of husband-wife couples are reported with inflammatory bowel disease who were seen in
the same geographical area in Nord Pas de Calais region of France and in Liège county (Belgium).
Among these 10 couples, four were concordant for Crohn’s disease, two for ulcerative colitis, and
four were discordant. In nine of 10 couples neither spouse had symptoms before marriage but
inflammatory bowel disease subsequently developed in both. In one couple, one spouse had Crohn’s disease before marriage and the other partner experienced symptoms afterwards. Eighteen children
were born to eight of 10 couples. Five of them developed Crohn’s disease but four belong to the
same family. In all cases the affected children were born to parents who both developed Crohn’s
disease after they had married and were conceived at a time when parents did not yet have
symptoms. It is proposed that this pattern of emergence of inflammatory bowel disease
suggests a role for an infectious agent yet to be identified.