It is obvious that sex has a lot to do with the human heart since it deals with raising of its heart beat just as it is with exercising. It might not be total abstenance from the act but it should be done with caution.
Basically the human heart is made up of muscles which requires great amount of force for it perform it primary functions, which is the pumping of blood. If this pumping of blood increase beyond normal level it could cause what is known as High Blood Pressure which could lead to death.
Let's take for example, someone that is suffering from heart problem engages in series of sexual intercourse, and the person's heart beat increases leading to high blood pressure. Well, the rest would be history for such a person.
And so it is not ideal for somebody suffering from heart disease to engage in marathonic rounds of sexual activities. It's better to seek the advise of a professional medical expert before considering doing so.