Males produce new sperm cells continually, so it's hard to see how radiation exposure could make them permanently sterile.
Females draw from the same stock of egg cells for their whole lives. The empirical evidence, from Hiroshima survivors, seems to be that even with a pretty hefty radiation dose, there is no epidemiologically detectable additional risk of birth defects. This seems to me to make sense, because radiation has the biggest effect on cells undergoing rapid division -- that's why it works as a cancer treatment. Eggs cells aren't undergoing division at all until fertilization.
There is some empirical evidence that low doses of radiation increase fertility in female humans, trout, and mice. According to one study,
Lightly irradiated rodents were more fertile than controls through several generations (increased ovulation in dams, increased number, viability and growth rates of young, and faster physical development of young) with no evidence of mutations in the young exposed in utero. Irradiated colonies were maintained in good health through 21 generations.
This is an example of a more general type of effect known as radiation hormesis, which is hypothesized to occur because radiation triggers cells' mechanisms for damage control. The doses at which measurable hormesis occurs are usually fairly large, so I would guess that any pro-fertility effect on ISS astronauts is negligible, and in any case effects such as increased ovulation, even if significant, would be temporary.