"Opposition to openly gay service isn’t rooted in something as subjective and ephemeral as “feelings.” It is rooted, instead, in human nature and a recognition of what drives young men to risk life and limb in the combat arms. Simply put, young men sign up for infantry duty in large part to prove their manhood. Gay men can be, and surely often are, great Soldiers. However, openly gay service is incompatible with the shared sense of manhood and masculinity that binds infantry units together into a brotherhood," - John Guardiano, FrumForum.
Compare with this quote from someone actually in the Special Forces:
“We have a gay guy [in the unit]. He’s big, he’s mean, and he kills lots of bad guys. No one cared that he was gay.”
Manhood is neither straight nor gay. It's male.