What an alarmist headline!  
At 0.3% {https://en.m.wikipedia.org/wiki/LGBT_demographics_of_the_United_States} w/in the general population (I would speculate lower w/in the 2.5 million serving) this ban will not significantly weaken our military. Nor is it likely to have a major impact on morale. 
Is Pres. Trump's tweet morally reprehensible? Yes! But as Gen. Mattis has stated, it is not an 'order' to the Pentagon. As usual, the President's tweets have created a tempest in a teapot; watch the other hand.