Why are women not allowed to join the Royal Marines?

Not that I want to join, my son asked me today&I couldn't answer. Wasnt there a young lady who took the 'test' just to prove she could pass? Un-bigotted answers only please, dont be offensive. Thank you:)
Update: Hannah!!! sounds like men have treated you badly!lol. men are not chauvanist pigs, theyre hot!LOL
Update 2: Especially Royal Marines hahahaha! wahoo!
Update 3: I didnt give you thumbs down Mr shady.....even though you're wearing your towel like a girl hehehe
Update 4: Nosdda- you belong with Hannah! You obviously know some pathetic women
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