Hasn't it always, always been 'Ladies first'?
No - I doubt Mr Ug held the door open for Mrs Ug in prehistoric times! (

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I wish I'd not asked about the order of precedence for men and women! I've just spent a very enjoyable half hour on the Debrett's website learning seating orders at dinner, correct forms of address, the order used by the Queen and the Lord Mayor of London walk through the City of London, etc. However there is no mention of ladies (or gentlemen) first.
I did find a section that reads:
"Certainly in a preamble to a speech, for example, at a function where a number of illustrious guests are present, it is not inevitable that the usual sequence of precedence is followed. Adjustment may need to be made in order to give due respect to a patron, president or guest of honour, and courtesy and common sense will overrule conventional good form."
and I wonder if that isn't how any variation could be justified
