What yall call onsies must be what we call "feet pajamas" lol
Onsies down here refer to a leotard-like shirt that snaps around the baby's bum. Yes, they make such shirts for women, and they seem awfully difficult to use the restroom in public!!!!!!!
Feet pajamas are for babies and adults, something similar to "long johns" or comfy one-piece pjs, we also called them blanket pajamas when they are fleece.
Ones for kids zip and have feet, adult ones usually button and come with or without feet, some have "butt flaps" for using the restroom but I usually just drop the top down. I like for my feet to be colder than the rest of me, so I like the foot-less kind, even though I still refer to them by what we called them as kids - "feet pajamas"
Sometimes I wear a thin tank top under them so when I drop them to pee, my top doesn't freeze, sometimes I just suck it up - the seat's gonna be cold anyway. LoL
My men (ex hubby and new fiancé both) have always found them funny-looking, or cute in a funky way, so I guess that's almost as good as sexy. Certainly not any worse than sweatpants and baggy tops!!