What is illicit sexual behavior?
This fault, which used to be called adultery and unnatural sex acts (arguably more narrow concepts), can be a useful fault ground. Unlike many of the other grounds,adultery can have no legal justification — proof of the act will establish fault. Adultery may be proved by direct evidence, including the declarations of the paramour and (since 1984) the admissions of the adulterous spouse. Adultery may also be proved by circumstantial evidence. The evidence of adultery, whether direct or circumstantial, “must tend to show both opportunity and inclination to engage in sexual intercourse and . . . when the evidence shows no more than opportunity, an issue of adultery should not be submitted.” Opportunity is some situation or circumstance in which the party and the paramour are alone and unsupervised; and the setting would suggest amorous activities. Inclination is the expression of feelings of love and affectionate behavior between the party and the lover. The evidence of opportunity was abundant in one of the well-known reported cases and included the following: observation of husband and woman leaving his farm house together at 10:30 a.m., their entering a residence together and staying for two hours, eating together in a restaurant, entering the same motel at separate times with both their cars remaining there throughout the night, driving together, and spending a night together in husband’s condominium, entering and leaving the condo separately. Those facts were not, however, sufficient to prove adultery because there was insufficient evidence of inclination. If the case contains such facts of opportunity without evidence of inclination as well, these facts may still be sufficient for establishing indignities arising from a spouse’s association with a member of the opposite sex.