More so than, say, HIV, TB and AIDS which are rampant in the male homosexual community, not to mention a whole host of other serious STDs? Don't forget the problems with blood supply and contamination of other members in a combat environment.
Let's face facts; homosexuality is an aberration and a mental illness with those suffering from it much more likely to have other serious mental afflictions and much more likely to succumb to substance abuse.
The US Armed Forces are not a social experiment. I think you can envision that 2 people who are sexually attracted tto each other may lose focus of the mission when the SHTF.
Number one, the worry about the diseases you mentioned would be nipped in the bud upon blood testing let’s say every six months or so. I am sure the military checks for these things when the recruit enlists. Not that a condom is 100% cure-all, but it will prevent a high degree of infection.
As far as substance abuse among the homosexual community. Yes, I agree with you to a degree, because it is a subculture that uplifts partying, clubbing and all that stuff. Alcohol abuse (esp) is quite high in the military among straights-—drugs, they just boot you out if they find out.....or maybe send you to rehab if Uncle Sam has invested a lot in the soldier.
The mental illness part is debatable. Now is their homosexuality in and of itself a mental illness (abnormal in the mathematical sense of the word yes, but not mentally ill in its common meaning) Do homosexuals suffer from a higher rate of other forms of mental illness than the population at large—ie schizophrenia, paranoia, OCD, manic depression etc-— from what I have seen yes, but a large part of that is due to other factors in their lives not directly due to their sexual attraction.