In 1959 underwear is white, at least for males. For females pastels are optional. Early in adolesence, Jane's first bra would have been a "training" bra, though what is being trained would not be specified. Snavely is bare-chested here since T-shirts are now outerwear, no long called under-shirts.
Snavely's shorts are of the type called "briefs," which he unconsciously associates with rabbits, since recently reading this passage in "Rabbit Run" by John Updike:
Old-fashioned masculine boxer shorts give way to tight briefs, somehow suggestive of girls' underpants. Or little boys' undies. Another step in the slide toward unisex.
As he ("Rabbit" Angstrom) comes back across the room Ruth laughs from the bed. He asks, "What's the joke?"
"In those damned underclothes you do look kind of like a rabbit. I thought only kids wore those elastic kind of pants."
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