For something that is simply a "T" shirt with attached snap crotch panty it can be fashionable worn by either sex.
Customarily worn by a toddler for ease in diaper changing there is not much sex to be expected here.
However worn by a shapely female it can be attractive in that respect. An athletic build male can also look good.
Various designs of the top of the onesie - polo shirt - mock T neck - turtle neck - sleeveless - cap sleeve - long and
short sleeve as well as a singlet. Then also the design of Little for Big with pull on skirt is an eye catcher - image
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Yes keeping the shirt tail in check is one feature and reducing the draft potential too.
Use in conjunction with diapers onesies offer critical support for the loaded "D" and the ease for changing.