Then I envy you that. Astartes have no children, except through the passing on of our gene-seed. We have Progenoid organs implanted in each of us, that when implanted within Aspirants, begin the transformational process which makes a youth into an Astartes. But by that time... the Astartes from whom the organs progenoids are harvested are most likely dead, though the surgery can be done while we still live, battlefield extraction is not uncommon.

[he looks regretful] Given the nature of my probable death, and the traitor status of my Legion, it is very unlikely anyone will ever carry my geneseed.