I had the same thought.  Probably what they're referring to is the fact
that generic drugs don't have to be as precisely measured as brand-name
drugs.  That is, the generic Viagra, for example, would only have to
have 80 mg of active ingredient in a 100 mg pill.  If it had 120 mg, it
might be harmful, I suppose.  But I doubt it.

Perhaps the counterfeit Viagra sold in other countries is even less
precisely measured.  But even then, I can't see it causing impotence.
It could cause a heart attack, or some other cardiovascular event; but
impotence?

On the other hand, if it contained impurities, or other products
instead of Viagra, who knows what the sequelae might be.
But yeah, it does sound like scare tactics.