You many not be aware of this, but U.S. military personnel are the most screened segment of American society for HIV. Tests are mandatory once a year, and generally before any deployment. Me and my team averaged an HIV test every 9 months or so. No organization in the United States has HIV transparency to the degree the military does.
From a medical standpoint, you'd be a fool to turn down a blood transfusion from any U.S. troop. I'd take one over any civilian. You don't know where they've been.
An infection can occur during the nine months between tests.
For that matter, blood transfusions in the field are extremely rare these days. Troops are medevac'd very quickly these days, in comparison to other wars. I don't know that the field transfusion is actually still done, in the sense you're thinking, although possibly in the special ops community in unusual conditions. With the advent of combat lifesavers and individual and team IV bags, that's the preferred method to restore fluids in the field after major trauma.
So, your odds of contracting HIV from a gay soldier are practically nonexistant. As I said, there are plenty of good reasons to keep gays out of the military, but this concern is quite overrated.
From the National Institute of Health:
EARLY SYMPTOMS OF HIV INFECTION If you are like many people, you will not have any symptoms when you first become infected with HIV. You may, however, have a flu-like illness within a month or two after exposure to the virus. This illness may include
Fever
Headache
Tiredness
Enlarged lymph nodes (glands of the immune system easily felt in the neck and groin)
These symptoms usually disappear within a week to a month and are often mistaken for those of another viral infection. During this period, people are very infectious, and HIV is present in large quantities in genital fluids. http://www.niaid.nih.gov/factsheets/hivinf.htm
In short, the tests given prior to deployment PROVE how concerned the military is with their walking blood supply.
Also, the promiscuity of gays AND their unbelievable willingness to find and engage in risky sex in every location means that they could be infectious with a month of deployment.
AND WE HAVEN'T even considered that the Gays are infamously infected with a host of other diseases that are not even checked for.....to include other blood-borne pathogens.