It seems you’re more likely to die from a heart attack when having sex while having an affair, than during sex with your regular partner, although this seems largely to apply to men.
A case report in the Journal of Forensic and Legal Medicine reports on the death of a woman who had a heart attack during extra-marital sex, something unusual in women. This is not conclusive evidence for the link between heart attacks and affairs in itself, of course, but the article reviews some suggestive evidence about sex, risk of death, physical and psychological stress.
Recent studies dispute early popular belief that sexual activity necessitates extraordinary physical effort, by showing that in normal settings, healthy adults show only mild to moderate increases in heart rate and blood pressure. However, this is not the case in individuals suffering from cardiovascular pathologies [heart problems], in which the relative risk of myocardial infarction [heart attack] in the two hours following sex increases 2.5-fold over the baseline. Furthermore, all reported cardiac deaths surrounding sexual conducts involved extramarital sex, suggesting psychological stress as an added factor.
However, as there is no data about behaviour, it’s not clear that psychological stress is the primary thing that increases risk, or whether people are just doing more strenuous things in their extramarital trysts.
The paper does mention one study from Israel, however, that sets the scene, and largely rules out the fact that this may be due to taking Viagra, which can put a strain on the heart
Between the years 1999 and 2008, the National Center of Forensic Medicine in Israel (population of 7.4 millions) investigated at least one case annually of sudden death of a male engaged in sexual activity. All these events took place with substantially younger women, in extramarital settings… In only one instance the toxicological examination revealed traces of sildenafil [Viagra] in the blood.
In all of the cases in this study, the cause was sudden heart failure in men with coronary heart disease.
Link to case report on PubMed.
Mind Hacks 
As Bart would say “well duh”