So they’re saying that Top Gun … isn’t top gun? 
But seriously, this is just a theory for now, probably a good theory, and there will need to be some sort of measures taken during long-duration spaceflight to alleviate it. But it really is still just a theory. From the abstract -
Oxidative stress and endothelial dysfunction are causative factors in the pathogenesis of erectile dysfunction, although the effects of spaceflight on erectile function have been unexplored.