I don't really want female marines... *taps his fingers together and looks at his feet* I'm sorry, I don't want to seem like a curmudgeon but really to me, it doesn't add anything but personally it detracts.
The current old fluff dictates that the organs and surgery only function in a male, and changing that would be a really big retcon but it would detract from the tragedy of being a space marine.
When you are taken by the men of the stars to face the trials and don the armor of the battle brothers, you give up everything, who you are, who you were, you will know nothing but hardship and war. No peace, no family, no love.
The video game 'Space Marine' does a good job of illustrating how some Marines look upon humans. They viewed the gaurdsman in a sympathetic light and there was a feeling of needing to protect them as they died in easily two or so hits from any greenskin. They are so far above us we're like children, the only children they'll ever have.
Don't get me wrong, I like well written strong female characters. Hell, Lt. Mirra from the above source was a wonderful character. She managed to hold the guard together by herself after the deaths of her commanding officers, which in an army in fluff that is usually regulated to the role of 'damsel in distress' is remarkable.
But I just don't think they would fit in the astartes as well, not without taking away some of the tragedy.