They shouldn't. Men and women are built differently, and have different shoes because of this.
Men, women and children all have differently-shaped feet; shoes are designed specifically for their respective types of feet, so even if a shoe is the correct size it mightn't necessarily be the correct shoe type for the person concerned.
When it comes to basic non-fashion styles such as slip-on shoes, thongs, boots, sandals and so on, it probably makes little difference.
Other types of footwear - fashion shoes, and so on - could cause problems. If a man wears a fashion shoe designed for women, or vice versa, the difference in structure could well cause them to automatically adjust the way they'd normally walk or stand, learned from years of wearing a quite different type of footwear, and this could result in muscular and skeletal problems if worn for any length of time, possibly causing great discomfort after only a matter of hours.
Men who enjoy cross-dressing, transsexuals, and entertainers who wear women's clothing for professional reasons, are practised in wearing women's shoes and careful in the styles of footwear they choose, allowing for the fact that men are generally heavier than women; they'll experience fewer problems, but still need to be sensible about what they wear and for how long.
Why a man might want to wear a woman's fashion shoe, of course, is another matter. Women's fashion shoes are not necessarily designed for the comfort of any woman, let alone a man. Possibly a man might want to find out what a woman endures for the sake of meeting current fashion expectations? He'd save himself a lot of discomfort simply by asking a few women, carefully selected from those he knows to be truthful.
He might like to allow a bit of time for this, because they'll quite likely also want to tell him how it feels to wear a bra all the time, or squeeze into jeans which are too small, or perform the contortions necessary to get inside a pair of tights. They might even want to share intimate information about what goes on in beauty salons.
In tropical Australia, men - surf lifesavers, and so on - sometimes wear tights in the ocean during sea-wasp (stinging jellyfish) season, as this helps protect them from the stings. I haven't met one who actually likes putting on the blasted things. I've never asked a surf lifesaver if he enjoys wearing women's shoes, but must remember to do so one day, and if I survive I'll post his answer here.
Since junior shoes use the men's sizing scale, if you're a woman's 8, you're a men's (or junior's) 6½.
If you wear an 11 in women's shoes, then that is equal to a 10 in mens.
Nine and a half.
Men should wear speedos and woman should wear bikinis and bras
Approximately a 13.
Crew length socks.
Since men wear shoes for longer periods, they should be taken proper care of. Regular polishing and dusting helps the shoes to remain in a good condition. Men's shoes are generally sturdy and last longer if they are kept with care. They should not be worn during the rainy season.
you would wear an 8
13
they wear moccasins
YES!
You subtract 2 from the woman's shoe size to get the man's shoe size in length. Since a man's and woman's heel are shaped differently, you may not get a good fit.