Because in a word, it's "divisive" - the last thing you want in an organization that needs to be "cohesive".
In my unit, we all agreed(I was a foolish 18 year old at the time) that if we ever identified someone of our unit as a homo, they would get our first bullet.
I know that pact gets passed on in every combat unit everywhere.
We were concerned that the sex drive of the queer would over take him "Brokeback" style if we were in a fox hole with them. We did not want to have to watch for a sex-crazed homo shooting us so he could get off on our dieing corpse. I know that not all homos are necrophiliacs, but at 18, I didn't know that, and I did not want to take the chance that the one in my foxhole was.