Well, using that logic, we should kick all radical leftists out of the military instead of just gays and lesbians who are radical leftists (not ALL are, you know).
Further, using your logic, what if FDR had kicked all racists out of the military during World War II? Weren't we better off unifying against the real enemy in Hitler, imperial Japan and Mussolini and saving the fight against segregation, racism and communism for another day? You can't fight all of your enemies at once. You have to pick the greatest threat of all and attack it with everything you've got. We won World War II because we didn't allow ourselves to get distracted from the immediate threat that had to be dealt with. Radical Islam is the immediate threat that we must deal with. Any soldier who doesn't agree with that statement should be kicked out of the military.
Yes, we should!
They pledged to defend the Constitution!
Being a radical leftist means being an enemy of the Constitution. Communists have nothing lost in the military.