Having lived in Europe and of course being an American, I would say the following about American and European attitudes towards not just speedos but clothing and sexuality in general.
We, meaning of course Americans, ARE prudes. Our society is conservative in almost every way, especially in regards to anything that even comes close to resembling sex or sexuality. People who have not experienced both American and European societies often don't understand that there really are fundamental differences even though by and large the US and Europe have similar racial makeup, similar religion, and even though much of Europe doesn't speak English as their primary language, English education is cumpolsory in most places in Europe so most Europeans are if not comfortable with it, at least exposed to the language.
European society is much more liberal than American. I don't know what it comes from, but I know that it's true. As a result any question that can be interpreted in a conservative/liberal light will be slanted to the conservative side by most Americans and to the liberal side by most Europeans.
My own opinion. I like speedos. I like how they feel on me, and I like how they look on men who have nice bodies, and especially on men with big cocks. But that's just me. I own three of them, but I also own a couple pairs of trunks too because I don't want to be the only guy at a pool wearing speedos.
Richard