Zappiel said:
Only if you approach them as purely gene-enhanced soldiers. I can't view them as such because of their origins. The genetics of the Emperor are beyond human (as we actually know from the background book Xenology that psykers have a physiological/genetic component to their abilities), and from the Emperor the Primarchs were made, using a mixture of science and sorcery. The Emperor himself is orders of magnitude more than any normal psyker - a lifespan of some thirty-nine thousand years at the time of his internment into the Golden Throne, a capacity for knowledge and understanding that exceeds the greatest augmented minds of the Mechanicum of Mars, psychic power sufficient to pose a real challenge to the Ruinous Powers and a presence such that no human being who ever met the Emperor has ever doubted the simple fact that he was the Emperor.
The Primarchs themselves possessed supernatural capabilities, and were each in some way psychic - Magnus the Red's psychic mastery is well-known, as are the precognitive sight of both Sanguinius and Curze, but less so is Corax's ability to walk unseen, or even the raw and awe-inspiring presence that each Primarch possessed to some extent.
The Astartes are created from the genes of the Primarchs. They are the result of incredbly complex genetic engineering the likes of which human science had never before seen, yet designed to be mass-produced in a way that previous genhanced soldiers (such as the Thunder Warriors or Adeptus Custodes) could not be. Each Space Marine is the fusion of carefully-selected human beings (chosen for physiological, psychological and genetic compatibility) and a series of implanted organs that automatically retroengineer the body and mind of a neophyte through processes developed during the dawn of the Imperium. The science behind each implant is beyond the capability of almost any human or posthuman mind to comprehend (many have tried, none have successfully replicated the Emperor's work), and each implant exists only because of the fusion of science and sorcery that created the Primarchs.
In essence, defining what makes a Space Marine a Space Marine is a lot harder than it looks, and is far from being a matter of pure science.
As for the matter of the interaction between the Warp and reality… the Warp presses close to reality in many places, and the mere existence of the Ruinous Powers demonstrates that the veil between is not as impermeable as might be comforting to think. The Warp impinges upon the fabric of reality every moment of every day… it is an influence that cannot be easily discarded.