Gay sex is physically destructive. While a horny heterosexual man in the military may be a walking collection of STDs thanks to his contact with the prostitutes who cluster around the military (as they have done since time immemorial), his bodily integrity will remain intact. He won’t have fecal incontinence, rectal bleeding, prolapsed rectums, mountainous hemorrhoids, etc. And given that his job is to fight, that bodily integrity is good enough for the military.
Graphic. And obvious.