As mentioned above, if a man uses steriods it cannot cause birth defects to his offspring because the steroids cannot affect the dna from his sperm. He will probably be much less likely to conceive a child because of a lowered sperm count, but if he does conceive a child, it won't be affected.
I believe steroids could negatively affect a woman's fetus if she used them while she was pregnant, but I'm just speculating, I have no hard information to back this up.