The Amentians are a sentient race living on a planet that's just a few stars away from our own. Mentally and physically, they're very similar to humans (and consequently have a sense of anatomical modesty similar to our own). However, there are several significant differences, particularly regarding their sense of sight.
An Amentian's attention is readily drawn to any strong visual stimulus. It's such a strong instinct that less conspicuous objects nearby can easily go completely unnoticed. Their eyes can also perceive a slightly different spectrum from our own, and this allows them to see through many solid objects.
There do exist materials suitable for manufacturing clothing that would appear opaque to them, but they are rare and difficult to manufacture, and during the early centuries when their cultures were developing, these were almost entirely unavailable. Instead, it is traditional for an Amentian to intentionally wear flashy and colorful ornaments on the parts of the body that are the furthest from those they wish to conceal. Colored ribbons in one's hair, gaudy bracelets on both wrists, and decorated ornamental boots are all common.
This is not to imply that Amentians do not wear clothing; it would be uncomfortable (and potentially dangerous) to wear nothing at all in many environments. They may wear trousers and shirts, or robes, or one-piece bodysuits as appropriate. But these things are merely functional or fashionable; they are unrelated to concerns of modesty.
If an Amentian were to wear underpants beneath his or her trousers, it would be seen as an inappropriate attempt to draw others' attention to the genital area. To add a belt with a large, bright buckle would be the height of sexual provocation. On the other hand, Amentians who are caught naked do not try to cover themselves or hide behind things. Instead, they stretch out their arms and wave their hands vigorously, in order to draw viewers' attention away from their nudity.
These differences will cause a great deal of confusion and embarrassment on both sides when we finally achieve first contact several years from now.