having low testosterone levels has nothing to do with having kids. it isn't testosterone that fertilizes an egg, it is 1 sperm.
having said this yes if you abuse steroids over time your sperm count gradually gets lower and lower. i tell guys all the time who are worried to simply make an appt and get their sperm count checked.. it is that easy
plenty of steroid users have had unwanted kids, like i said it takes 1 sperm. your odds are less of course, but 1 means one