How about a Genetic loophole?
I remember reading in some thread somewhere (someone had some sciency explanation) about genetics being a reason for only male space marines and female chromozoms being incompatible with all the genetic augmentations. That and the difference in physical strength due to much lower levels of testosterone.
Anyway, here's my loop hole: http://en.wikipedia.org/wiki/Complete_androgen_insensitivity_syndrome
Essentially, the person is female, but not. The syndrome above bassically says the ovaries turn into testicals and produce testosterone. But no other major changes happen (cept no menstral cycle, and the increased testosterone would allow her to be much more equal physically AND have male chromozomes ). Rare cases there are physical problems, but this is make belive so we'll assume best case scenario (they actually had this happen on House M.D. where a super model had this problem + testicular cancer).
But anyway, you could argue this point in making it possible, as the female would be compatible in every way genetically and strength wise. That way all of the canon nazi's can still sleep at night. : D
Although personally, I'd just let them play a female if it was an issue. Wouldn't hurt anything, but it might make for funnier RP situations (or at least give more veriety of options).