The military is currently plagued by an epidemic of sexual harassment which eats up tax payer money and, more importantly, affects women and men with a strong desire to serve our country. The ideal of co-ed military units is just that, an ideal, something to strive for, but clearly not feasible.
Men and women are driven by by a primal urge to procreate. The only way to remove this desire, short of castration, is to physically separate the two genders.
I am not stating women are not equal to men. I am just saying that some human beings obey the orders of their loins, not the orders of their chain of command. And even when consensual, sexual relationships between two members of a unit can harm mission readiness and morale.