Though not a historian, I think it is safe for me to say that gays have been a part of humanity for a long, long time. Each culture/society throughout history has provided different levels of acceptance for homosexuality. For many western cultures/societies it may seem like there are more gays (both men & women) now then there used to be in the recent past. But, I think that is more a function of great acceptance and visibility in traditionally Judeo-Christian cultures/societies, rather than a larger number of gays being born.
If you believe the concept that at least 10% of the population today is gay, and you accept the above, then it is reasonable to assume that 10% of the population 10 years ago, 100 years ago, etc... were also gay. This means that throughout history gays have served successfully -- often with distinction and bravery -- in the military, whether their fellow soldiers have known it or not. Therefore, there is ample proof that gays are not "detrimental" to unit cohesion or any of the other bollox that you hear.
The problem with gays in the military is twofold, and neither has to do with gays themselves. One part is prejudice, pure and simple. As a species, we tend not to like people who are significantly different than ourselves in some way. This is even more so, when the people who are different occupy a minority group. For gays in the military, this means that once "out" to their comrades, they will often be looked down upon as now "less than" they were before.
The second issue, I think is stereotypes that are held by many people, especially straight men, about what it means to be gay. Many people think gay men find every other man (gay or straight, fat or thin, old or young, etc...) attractive. In other words, they think, if it has a dick and is breathing, we'll jump on it. Plus, they often think that we just are incapable of keeping "it" in our pants, 'cause all we think about is sex. So, they become afraid that, a.) Pvt. Gayboy will be trying to seduce and/or "convert" Crpl. Straightguy; and, b.) Sgt. Gayman and Pvt. Gayboy will, at an inopportune time (like the middle of battle), start having sex and cause their comrades to be injured or die.