Before I proceed, let me say that I believe the human cost of the war in Afghanistan, to all sides, is too great to justify continued US involvement in the region. Our initial reason to attack was better than our reason to attack Iraq, but we have worn out our welcome in any case, and I urge President Obama to return the troops to the continental United States as soon as possible.
Next let me assert that I do not consider rapes and battlefield deaths to be strictly fungible, only approximately equivalent. Soldiers do not volunteer to be killed; rape victims do not even volunteer for hazardous duty.* On the other hand, with love and support, rape victims can recover while KIAs (and many WIAs) cannot. (I count myself lucky; none of the young men I've known who've been to Iraq and Afghanistan were killed or injured. Welcome back, men.)
Finally, I'll grant that the race of a rapist does not really matter, in the sense that the crime is not legally mitigated or aggravated by race. We have been told, however, that "hate crimes" are by definition more severe than other crimes. I maintain that, without telepathy, hate crimes are indistinguishable from noneconomic interracial crime. Same-race rape, though obviously hateful in some sense, cannot be considered a hate crime as the term is generally used.
On to the data.
32,000 whites were raped by black men in 2006.** (This varies a little from year to year.)
We had 20,297 troops in Afghanistan in February 2006.*** (It increased somewhat by March of the next year.)
If we lost one soldier in Afghanistan for every white person raped by a black man, we would have lost all US forces in Afghanistan in under eight months.
* They can certainly take measures to protect themselves, but this is an active rather than a passive choice.
** The white-on-white rape count for the same year was a little over 98,000. Since we are told "predators like to hunt within their ethnic group", this figure is depressing rather than surprising. (White men rape far fewer than 100 blacks in a given year.)
*** The President, having been elected on a platform of change, with strong support from the anti-war movement, has changed our troop levels in that country to 94,000. (Here is Chris Roach on the subject.) If the black-on-white rape rate stays stable, the figure quotes above would rise from eight months to about three years.