My assumption is also that sexual relationships on the battlefield would serve as a distraction. Women getting pregnant serves as an even bigger one. Should the military put much resources into a woman preparing her for combat only to send her home when she's knocked up by someone in her unit. And then there is definitely the matter of men getting caught up in petty b.s. - which is their fault I'll grant you that - when dealing with sexual tensions.
Men and women are different. Women IMO are not made for battle. These differences are clear when it comes to size, shape, etc. Then there's the sexual manipulations and games. How many men do you know have legs as their avatar? There's just no room for that stuff on the battlefield. It wreaks havoc both on deployment and on the home front and the negatives out weight the positives.