Just to help with this all of the gene-seed was originally crafted in the 30th Millennium by the Emperor of Mankind in his Himalayan gene-laboratories deep beneath the Himalayan Mountains for this purpose using the genomes of each of the 20 Primarchs as a template. (Here is a little real science for you) In May of 2017 an article was published that "out of 20,000 genes looked into 6,500 of them are expressed differently in men and women." So that is more than 25 %. (back to the game) Since the First Founding, the Chapters of the Adeptus Astartes have lost the ability to engineer new gene-seed or implant organs from scratch. Now using the gene-seeds they are stuck with some chapters are lucky to have a 1 in 10 success rate other 1 in 100 and that is with males who's genomes are most likely to be compatible with the original gene seeds.
So there you have it even if you just did it on the math alone the ones that have a 1 in 10 using males(10% success) would be lucky to get a 3 in 40 using females(7.5%) and the ones with 1 in 100(1% success) would be lucky to get a 3 in 400(0.75%) using females so from a pure math position you go with the highest success rate. Not to mention if you go from a science stand point of the 6.5 K genomes that are different between males and females how much do you want to bet that there aren't a good chunk of those that are inherently different in the genders that are required to make it work so now you drop the success rate from 3 in 40 (7.5 %) success rate to 0.00000000000000000000000000000000001 % success rate or 1 out of 10000000000000000000000000000000000 because you will have that one freak of nature that is a female but possesses all of the required missing male genomes in there body as a mutation.
Sorry to say it but men and women are not identical that is why some medicine works better for women than men and vice versa, same for ethnicity, it is no one trying to be mean except maybe mother nature. So until someone succeeds in replicating the lost technology sorry it is not going to happen and it is still not going to happen because the Emperor (may his will guide my words) based his research off of the aborted Primarch project.
Now the Primarch project led to the creation of the Primarchs who were the twenty genetically-engineered "sons" of the Emperor of Mankind, and the genetic "fathers" of the Space Marine Legions. The Emperor used his own DNA in their creation. It is said they were created with the Emperor with the aid of forbidden sciences and arcane lore derived from the Warp. The Emperor utilized dark powers to construct the Primarchs. It is speculated the Primarchs were imbued with energies and knowledge that the Emperor gained in a Warp Portal on Molech.
So in order to rediscover the technology you will have to uncover the forbidden sciences and arcane lore in the warp which is deadlier now in 40 K than it was in 30 K, do this without the Emperors help of course because he is in no condition to go there, return back and get DNA from the Emperor which should be fun if any of it is still viable and you can get to the golden throne, genetically alter his DNA to give birth to a Female Primarch without corrupting it. Then use the genome of that Primarch to create new Gene Seeds, which by the way is also lost technology and needs to be rediscover then you can make a female space marine. Have fun. By the way the above is only about a 10th of what would have to be done if that. There was a lot more to making the Primarch's alone than what I have above the, the lore in Warhammer is so in-depth it makes real life seem simple so it takes quite a bit to alter Canon lore without alienating your current ensured fan base to such a degree that you lose all of them for an unknown fan base you may possibly gain but are not assured to gain.