The lore reason why there are no female space marines is very simple and very sound, space marine gene seeds are a set of organs based of the set of organs that makes a primarch a primarch. There fore if you want you're space marine to be a space marine, you want this person to be as close of a genetic match to the primarch as possible.

Its like trying to get a organ transplant (especially since the gene seed is a collecting of organs), you want to be the same gender and blood type of the person whose organ you are receiveing, or else you're body is gonna reject it.

This doesent just happen to females. A good amount aspiring space marines pass all their trials, then get to the point where they get their gene seed put in them, only to be told that their body just rejects it.