The main biological impact of low-level microwaves is oxidative stress damage and free radical formation. These effects are seen in most cell types after even low-level microwave exposure and can lead to DNA damage. Sperm cells lack the protective mechanisms other, more permanent cell types employ so the effect is more apparent in sperm. Its not a thermal effect, as the headline would suggest, as it happens at levels far below the threshold of RF heating. Thus: official safety guidelines are not protective and need to be revised to take into account non-thermal RF effects. In the meanwhile, reduce your exposure. For reviews on these effects see Yakymenko I. et al. 2015 and Pall M. 2013.