In the Warhammer 40k Universe there is no shortage of bad ass female characters. There are characters such as Lieutenant Mira from the Space Marine game which was ready to stand up to the Inquisition to back up Captain Titus of the Ultramarines.
There are also characters like Saint Celestine, and the entire army of Sisters of Battle. Unfortunately, unlike in the lore , there really are not many female models in the table top game. The Sisters of Battles make up most of the female models available in the game, as opposed to the lore where female warriors, or characters can be found through out the Imperium of Man.
Remember, in the Imperium of Man, so long as you can hold a lasgun you are welcome to join the forces of the Emperor to die for mankind. Space Marines are very different. So far all of the Primarchs which have been introduced into the lore have been men. Since Space Marine legions are created through the use of their Primarch’s genes, the assumption is that the initiates which are recruited into the ranks of their respective chapters must also be the same sex as their respective Primarchs. Since all the Primarchs are male, all their gene sons (the Marines) must also be male in order for their organ implants to be compatible. This is the reason we will never see female Ultramarines, Imperial Fists, Raven Guard, or any other current canon chapter. Does this mean that there can be no female Space Marines ever though? Absolutely not.
In the lore it is stated that all but two of the Primarchs were reunited with the Emperor, and incorporated into the Great Crusade. Who is to say that one of these two, or even both of these Primarchs were not female? If they were then we could make the assumption that the legions which they would have mothered would be all female legions. At the moment there is nothing in the lore to suggest there are any female Space Marines in existence. This does not mean that you can not field a female Space Marine army in game, in fact there are several third party suppliers which sell female heads which can be incorporated into your space Marine kits to create female Space Marines. You can also take heads from Sisters of Battle kits to create your own female Space Marine chapters, but it will be up to you to write your own lore for the chapter you create, as well as the Primarch which they inherit their genes from.
Third Party Bits can be bought – Kitbash away!