This is a nonsensical argument. Are you saying we're just a few vaccines away from you being okay with homosexuality in the military? I don't believe that for a minute.
Gays exist in the military now, and they'll exist in basically the same numbers if 'don't ask don't tell' goes away. There just aren't that many gays, per capita, that this number is going to vary greatly. Nor do I think that there's some vast number of gays lurking outside of recruiting stations, waiting for the day the Army comes around. Or that a vast number of straight soldiers that will turn gay once the Army gives the proverbial thumbs up. So your medical point is basically moot. In the closet or out, the numbers won't change, and the threat won't vary.
The real issue is that homosexuality is unnatural and innately rejected by normally functioning people. If you open the military to it, and force them to espouse it's value, you're going to drive out traditional minded folks who might be okay with turning a blind eye, but don't want to have to promote homosexuality as a function of their jobs.
Why don't we just stick to that?
Because the other issue is very, very real.
And it is a serious morale negative.
There is no vaccine for hiv and many say there never will be one.
As wagglebee says, would you get on a plane knowing that many of the parts might be defective?