So I am travelling in a few weeks to Barcelona, Cote D'azur, and to Genoa Italia and was wondering if men wear thongs on these beaches. I want to try it personally but am unsure how taboo it would be. Otherwise I will wear my speedo. I'm 22, good body, and want to just tan in a thong swimsuit.
To be legally covered (pun intended), just make sure that you have some piece of clothing around your scrotum and the shaft.
The social aspect is quite more complex and the acceptable degree of skimpiness will not just differ between cities, but also between the different beaches in each region. Even if male thongs are not too common on mediterranean beaches, except on strict nude beaches, where even a thong may be too much, I would doubt that anyone at all would bother if an odd guest is wearing one.