This is a question the Eugene Volokh asks, noting that many of the reasons offered for the exclusion of gays generally (if you accept them) really only apply to gay men - not to gay women.
The main justification we've been hearing for excluding gay men is that if the soldiers in an all-male combat units have homosexual relationships, this can lead to favoritism, jealousy, and tensions that aren't present if the soldiers are all straight men. But whatever merit this argument may or may not have, it surely doesn't apply to lesbians, at least unless the military starts having all-female combat units.
What's more, lesbians might actually be more effective than straight women, because I take it that they have a much lower rate of pregnancy; and in a military which is more male than female, they are likely to account for fewer possibly tension-inducing sexual relationships than heterosexual women. Also, if the concern is that homosexual men are more likely to be carriers of sexually transmitted diseases than are heterosexual men, I take it that this concern too will not apply to lesbians.
The exclusion of lesbians both uses up resources (for investigation and dismissal) and ignores potential candidates who might have a great deal to offer. But in exchange for what? In what way is the effectiveness of the military increased by the absence of lesbians? It's not a clear-cut case that the effectiveness of the military is improved by the absence of gay men, but the case against gay women is even less credible.
Maybe, just maybe, the reason for excluding gay women isn't really based upon military effectiveness? And if that is true, then the reasons for excluding gay men will be open to question as well. Can't have that, can we?