>>96359990
>The way the Emperor ensured space marines can't just reproduce normally was by making one of those modifications chemical gelding. Their balls don't work no more, which is also why they feel no sexual desire (unless Slaanesh modifies them)
This is some headcannon. It's simply never addressed, and on the few occasions that Black Library authors put Space Marines in provocative situations it's treated as a matter of disinterest borne of psychological conditioning. It's implicit to the concept that a fully functional pair of testes is required for the ascension from human to astartes, since the process is "keyed to male hormones."

Space Marines are explicitly subject to "psycho-surgery" and "hypno-indoctrination" that strips away everything but their capacity for violence. That's a much more plausible explanation for "why don't they have sex" than some castrati bullshit that's never attested to in the fiction.

And if you want to delve into the most likely outcome of a Space Marine attempting to reproduce sexually, you'd probably wind up with a fair to inseminate due to incompatibility from all the generic engineering, or a stillbirth if somehow the insemination did take - but I'm not here pretending that's anything other than a headcanon fanfiction theory.