I don't work in this field so I'm making a bit of a guess here....

I don't work in this field so I'm making a bit of a guess here. But the reason the use healthy males is because then the trial has less variables and can give reliable results. If they used unhealthy males, then they would all need to have exactly the same amount of illness/sickness/condition etc which would be impossible to arrange and then you wouldn't know if good or poor results are due to the trial drug/application or due to the illness of that individual. The reasons for ED are varied and can be either physical or psychological so having a trial with healthy men helps to remove variables and allows the drug to be fairly evaluated.