Vigorous exercise will not impair your fertility at all if it's
part of a healthy lifestyle. Some female athletes are
extraordinarily thin however, which can cause a condition that
prevents the hypothalamus from properly releasing
gonadotropin-releasing hormone (GNRH).
In turn, this can completely stop ovulation as well as estrogen
production. So an extremely underweight woman may be perfectly
fertile, in that she has plenty of healthy eggs in her body. But
she may be unable to release her eggs through ovulation because
she's stopped producing certain hormones. As long as you continue
to have your period (a sign that you are ovulating), your fertility
should not be impaired.
Vigorous exercise is also fine for men, as long as they don't try
to enhance their performance with anabolic steroids, some of which
are legally and readily available in health food stores in the form
of androstenedione. Androstenedione is a "natural" anabolic
steroid, but that doesn't make it any less potent. Like any other
anabolic steroid, it can increase muscle mass dramatically, but it
can also shrink the testicles, lower the sperm count, and cause
liver disease and early heart attacks.