Why are lots of men and boys embarrassed about wearing speedos?

Lots of male competitive swimmers wear speedos by choice, but many (perhaps most) guys who aren't competitive swimmers seem to really dislike wearing speedos, at least in UK/US.

E. g. when I went on holiday to France last summer with some friends, we found out the rules of the pool and water park at our hotel were that men and boys could only wear speedos, and our male friends did not seem happy about that haha. And when we did swimming in PE in school in from year 9 to 11 they also had that rule for the swimming uniform and the boys always seemed a bit embarrassed about that too haha. I think they changed the rule eventually because the boys kept complaining.

Why do (many/most) guys seem to dislike wearing speedos so much, and so much more than e. g. a short pair of swim shorts? Is it that the bulge is too prominent? Is it that the design is perceived as feminine? Do they think they look unattractive? (they don't by the way haha).

(In both cases I've observed there was a bit of teasing from the girls of the boys but only because they were already shy, so I don't think that can be the only reason)

Why are lots of men and boys embarrassed about wearing speedos?
Why are lots of men and boys embarrassed about wearing speedos?
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