But exactly what about cohabitation worried him was unclear.
I can think of a couple problems.
A straight and gay Marine are assigned the same quarters. The gay Marine makes an unwanted advance. A fight results. In the PC world who would be punished?
Two gay Marines are assigned the same quarters. They soon develop a sexual relationship. Straight Marines complain – why can’t their girl/boyfriends be their room mates?
Not the best thing for good order and discipline.
I can think of a few more, much worse problems, that have to do with open gays getting into the upper eschelons of the chain-of-command, as they surely will once the "door is opened." If they follow the model they've pursued in civilian organizations, expect pink ICBM bases, pink submarines, pink air-defense bases.
Expect our adversaries to know which is which, too.