That mix of clothed and dressed is everything. The fabric (or rubber or plastic wrap or macrame or whatever people where) caresses, highlights, partially exposes, hides, teases or shows off the body. And I want to see.
I get that it's something simple often, like a t-shirt and shorts. But compare the full nudity (which has its own definite charms) to partially clothed and you can feel the seduction.
Now that may be a seduction the guy has no intention of, or is otherwise totally unaware of, or it may just be as simple as him going out with an easy sex appeal, wanting to look good for himself and whoever ends up watching.
Or it may be me reading into a clueless guy's appearance. He's just having a purely functional day, taking his shirt off because he's warm (or maybe his buddy cut it off him while he was asleep). Or he accidentally wore a t-shirt that is about to explode from his pecs. Totally an accident.
Here's to more happy accidents.