To answer your questions:
1) I am not sure of the exact mechanism why benadryl can cause impotence, however it is not uncommon that anticholinergic medications are associated with erectile dysfunction. I do not believe that Benadryl has any effect on the penile vasculature.
2) Again, I cannot be 100% sure, but it is thought that yes, anticholinergics certainly can affect erections by their default mechanism of action.
3) Yes, I have seen several cases where OTC antihistamines affect erections.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
Medical Weblog:
kevinmd_b