Discuss @David Flin 's latest article here
It can be a bit less obvious than that. Women in combat aircraft today is not too much of a stretch; put that into a story set in the 1940s, and you need an explanation. The Misfit series has this, and it has an explanation for that. Without that explanation, that would be a simple matter of modern attitudes that wouldn't be appropriate.
Claudia Edwards once wrote an otherwise good book The Taming of the Forest King, in which men and women were in combat units in a sort of late medieval sort of period with added magic. The Colonel of the regiment was a woman, her second-in-command a man. The logic and consistency of the situation was maintained. Right up until the point where there was a Ball they attended. The second-in-command went in uniform, obviously. No problem. The Colonel of the Regiment wore a ball gown.