It's not a "natural instinct." It's entirely a cultural thing. It just appears inborn because of the way gender socialization works. Just like every other aspect of gender.
If it's not a natural instinct (which you couldn't prove anyway), you still admit that it's a cultural one, and therefore its practical implications are the same. Whether or not you believe it to be natural has no bearing on the impact a change like this would have on the military...
You could respond the same way to integration in the military and the repeal of DADT...
