Honestly, yes. I'm a long-time Games Workshop player, and the 'why are Space Marines men-only?' question has been around a really long time. GW has been very firm on the subject, and the reason is as follows:
The Adeptus Astartes, or Space Marines, are basically genetically modified supersoldiers who gain their abilities partly due to gene therapies, partly due to special implants grown from material taken from the Primarchs (the original Space Marine leaders created by the Emperor) and in part from intense training and hypno-indoctrination.
From the genetic standpoint, the implants were all created using the bodies and organs of the Primarchs as a template, and in addition those were watered-down versions of the Emperor's own body. All of it came from male templates and was designed for use with males and males only.
Now, of course much of this is a reflection of the time period in which the game was created, and many people over the years have complained- but in a move I respect, Games Workshop has stood firm on the matter over the years.
Mind, almost all the other Warhammer 40K armies have females- and in many armies the females are in charge- so I honestly don't think having one Army being 'men only' should be an issue. Indeed, there is an army which is basically 'women only' (the Adeptus Sororitas, or Battle Sisters) so I'd say the scales are balanced on some level.
As to why your idea, as stated, is unworkable:
Space Marines, in general, do not Breed. The Canon has never come right out and said whether an Astartes has working reproductive organs, but the implantation procedures, training, and the implants themselves tend to indicate that while the body is reinforced and supported for War, non-essential organs (cough cough) were not enhanced or supported at all.
The entire Space Marine creation process is formed around implantation, none of which would be passed on via insemination, so at best if a Space Marine *did* impregnate a human woman the child would be genetically based on whatever the Space Marine was before being chosen to become an Adeptus Astartes. At worst, the sheer amount of physical changes that have taken place in the Space Marine would at the very least render them either sterile or screw up their 'normal' genetics so badly that they probably would not produce a living child.
Now, your story *is* viable in certain ways. Many Space Marine Chapters, if not most of them, have normal humans who work for the Chapter, from Archivists and Clerks to workers in the Chapter Monasteries. Many Chapters choose those servants from the planets they recruit from, and it's also not unheard of for higher-ranked Marines to have proteges or take normal humans under their wing to a certain extent. The thing to remember, though, is that the unifying factor of any Space Marine Chapter is that, at heart, they are Marines. Marines are chosen from a young age, put through horrific training regimens that most don't survive, and when they are finally chosen it's a huge honor. I won't even get into the fact that the implantation processes themselves have been buried in so much tradition that a lot of neophytes die in the process due to unneeded rituals that actually hurt the survival rate of the process.
However, in the end- if you have survived the process and become a Space Marine, that's all that needs to be said. Space Marines all share that beginning, from Scouts to Chapter Masters, and that puts them so far apart from normal humanity that there is a huge rift in understanding between them.
Sketchy, the basic issue you're facing is that honestly, this issue has been tried many times. I am not judging your particular idea, but many...many...previous attempts at this type of story have been, basically, an author trying to change basic Canon in an effort to be unique or tell the story of 'that one, messiah-like exception'. The problem is that once that exception is made, it is an exception that can be used by others, to the point where the exception becomes the rule. And that hurts far worse than it helps.
