In the US military, rape, violence and sexual harrassment figures against women are massively disproportionate to those in everyday life. Women who serve are at more risk of attack from their colleagues than the enemy.
From what I understand, the UK is following suit.
One of the factors cited regularly is the unwelcome display of pornography.
Hanging calendars like this in a workplace is hostile and threatening towards women who work there. Good luck arguing it out with your HR personnel in any other type of workplace. Why should an exception be made for the military where trust, teamwork and respect between colleagues is of critical importance?