Let's take them in reverse order. The biology that matters here is deep at the base of your brain, not in your pants. It's the amygdala, the section of specialized cells the size of the top of your thumb (size does not matter here) in the limbic area of the brain, responsible for detecting and responding to danger. It's responsible for one of two basic genetic imperatives, survival. The other is making copies of yourself - reproduction - but this study suggests that at least in some circumstances, when it's a question of one or the other, for some people survival comes first and the amygdala calls the tune. (Most of the heart attack survivors in the study were around 60 years old, so reproduction may have been less of a subconscious driver for sex. Also, to be fair, if 30% of men and 40% of women reduced the frequency of sex after surviving a heart attack, 7 in ten men and 6 in ten women didn't.)
     Even more tellingly, the study found that among those who talked to their doctors about sex after heart attack, either upon discharge from immediate medical care or during the year afterwards, far fewer feared fooling around. What's interesting about that, from the standpoint of risk perception, is that a sense of control, based on greater knowledge, generally reduces fear. In other words, the statistical likelihood of sex triggering a successive heart attack was the same for both the informed and uninformed patients. But the informed ones were less afraid because knowledge is power and power proves a sense of control, and control is an important psychological factor in just how afraid we are, or aren't.
      One other interesting finding in the study, which probably won't come as much of a surprise. Sex was a higher priority for men than women. Twice as many male heart attack survivors asked their doctors about what the heart attack meant for their sex life. Also, women feared sex-after-heart attack more than men. This may reflect on studies in the risk perception literature that identify "The White Male Effect", in which white men between 18 and 59 are generally less afraid of the same things than white women or people of color or either gender. (One explanation for this could be the fact that control is an important factor in risk perception...the more you feel, the less you fear. Women and people of color may feel less control over things, in general, than white males.) This gender difference may also reflect on the interplay between fear and libido, with many studies finding that the psychological circumstances around sexuality matter more to women than men. Fear of dying would be a definite turn off. Or it may be more anatomical than psychological.
     It's probably some of all of the above. But it's safe to say that the amygdala, and fear, are powerful forces, even powerful enough to diminish one of the most primal drives of all.

David Ropeik is an Instructor at Harvard and author of How Risky Is It, Really? Why Our Fears Don't Always Match the Facts