>>8893002
Assuming the modifications made on their biology by the augmentation organs don't affect the sperm, there's no reason why Astartes couldn't be fertile. It would just be a regular kid, though, not an astartes.

The main issue when it comes to sexing astartes is minimal urge and minimal time. Space marines are always busy and it takes slaaneshi-tier cockteasing to get them horny.
Unorthodox chapters like Space Yiffs would be possible exceptions to this limitation.