Well the gene problem only explain why since all primarch are male space marine are male.. By the way "creating" a space marine does include base upgrade but the entire process is centered around transforming the aspirant into something reassembling the primarch. That why the sons of horus ressemble him à lot, why most thousands sons are psyker and why the blood angel can still feel the death of their primarch. So in all that technicality we aren't talking about WHY all the primarch are men. Because, in my opinion, primarch where made by the Emperor "in his image" using his gene and each represent a part of the emperor. An aspect of his human psyche. The ultramarine being his pride, the lunar wolves his ambition the thousands sons his desire for knowledge and power The space wolve being his animalistic nature ect ect The primarch are more of a deformed copy of himself than of a real sons. That is a reason why he never really treat them as sons, why he has complete confiance but also don't estimate them much.
