I honestly do not think it should be legal to advertise any form of medicine whether pharmaceutically approved or alternative medicine.
Anyone needing Viagra would be well advised to start by going to his doctor - no-one else can judge whether it is advisable or not.
Standards vary so much these days, that it must be hard to legistrate on public decency.
I have seen advertisments for products such as contraceptives, medicaments for prostrate trouble, sanitary protection, sun creme or bathing suits that I found distasteful.
Others may well not find any of the above embarrassing, so it is a difficult thing to judge.
It should be possible to make an advert that conveys the message to the age group most likely to neeed Viagra, which a small child wouldn't fathom and with some form of visual content that is unobjectionable.