1. Good order and discipline. Sexual relationships among troops cause unforeseen deviations from normal patterns of unit support and cohesions. We don't want our fighting men and supporting women housed with those who sexually desire them.
When sex desire enters the equation, trust is destroyed. When one person wants sexual gratification from another, immediately ulterior motives, duplicity, special favored status, envy, anger, and other very divisive emotions grow. How can a group of fighting men exist with this going on? That also a reason why men and women mixing as they do is not a good idea. Especially in sleeping quarters, showering and so on.
Also, if, for instance, there's some sexual attraction going on between a man and a woman, the man may not reciprocate, but not feel necessarily threatened. But imagine a situation with a homosexual man lusting after a normal man, put that in a frontline or other dangerous situation! Sounds like very bad trouble.