I couldn't get past the first page without wanting to crucify this thread so pardon me if anything or everything I'm about to say is redundant.
1) SISTERS OF BATTLE ARE NOT SPACE MARINES! Yes, they do wear power armor and have bolters, but they are not genetically enhanced in any way and do not follow the same customs as the Space Marines. They are basically beefed up Imperial Guardsmen. They have the same basic stat line of Imperial Guard except the upped armor save.
2) There is a reason that you never see women Space Marines. Go back about 600 years to the Medieval Ages. What did women do in those times? Go out and fight the wars, work in the local blacksmith? No. They farmed and birthed children, preferably SONS to go out and fight wars and work towards the betterment of society. Women were taught to stay in the house and take good care of it and await their beloved husbands and sons to return home. Even 300 years ago in Native American society women stayed home, farmed or gathered herbs, and birthed children while the men went hunting and fighting. That is the way society had been, and those are the principles that the future societies are built on. The Space Marine Chapters take all their potential recruits (save the Ultramarine, but they're a completely different story) from cultures that are always fighting and take their best young warriors. Women do not spend much time fighting in savage warfare they are generally considered "too squishy" for this to be a probability.
3) Yes, Space Marines are loaded down with ridiculous amounts of heavy armor. This is compensated for by not the abnormal strength of the Space Marines, but by the fact that the power armor they wear has cybernetic "muscles". The armor moves with the wearer instead of fighting against them (due to the black carapace) so it doesn't feel like they're wearing 200 lbs. of armor.
4) There were female Necrons. You just don't know that because their entire race was cast into the robotic skeletons to gain eternal life in service of the C'tan. No, they dont look different because they're not wearing armor. That is literally their body.
Lastly, and this is the most important thing i will say in this thread (probably), is that nowhere in the Warhammer 40,000 fluff does it say anything about WOMEN NOT BEING ABLE TO BE SPACE MARINES. Nor does it say anywhere that women can not accept the genetic implants of the Space Marines. If you make the fluff for your own army (let's say, maybe your Space Marine Chapter is from a "reverse society" where the women are the fighters and the men stay home and do the dishes or whatever. That way the women would be the potential recruits for that home world therefore your entire chapter would be made up of women), you can do whatever floats your boat with your army. Yes, you can have women Space Marines and point this out to anyone that ever tries to argue against that. Then give me their address so I can stomp their face for being retarded.