Because biology matters:

MOTHERHOOD begins as a tempestuously physical experience but quickly becomes a political one. Once a woman's pregnancy goes public, the storm moves outside. Don't pile on the pounds! Your child will be obese. Don't eat too little, or your baby will be born too small. For heaven's sake, don't drink alcohol. Oh, please: you can sip some wine now and again. And no matter how many contradictory things the experts say, don't panic. Stress hormones wreak havoc on a baby's budding nervous system.

All this advice rains down on expectant mothers for the obvious reason that mothers carry babies and create the environments in which they grow. What if it turned out, though, that expectant fathers molded babies, too, and not just by way of genes?

Biology is making it clearer by the day that a man's health and well-being have a measurable impact on his future children's health and happiness. This is not because a strong, resilient man has a greater likelihood of being a fabulous dad - or not only for that reason - or because he's probably got good genes. Whether a man's genes are good or bad (and whatever "good" and "bad" mean in this context), his children's bodies and minds will reflect lifestyle choices he has made over the years, even if he made those choices long before he ever imagined himself strapping on a Baby Bjorn.

Doctors have been telling men for years that smoking, drinking and recreational drugs can lower the quality of their sperm. What doctors should probably add is that the health of unborn children can be affected by what and how much men eat; the toxins they absorb; the traumas they endure; their poverty or powerlessness; and their age at the time of conception. In other words, what a man needs to know is that his life experience leaves biological traces on his children. Even more astonishingly, those children may pass those traces along to their children. -- The New York Times