Wrong. The genetic engineering doesn't touch the gonads.
In fact, there's surprisingly little actual genetic engineering involved. The most genetic engineering that happens is with the occulobe where it alters optical functioning. The rest of it are all implants that function separately from and with little regard to the larger human as a whole, which is why they require all the other organs to maintain a correct hormonal balance and function. The reason why they get them young is because the process is an incredibly hormonal one and naturally it works best in puberty when the human body is already going through a hormone storm and the massive changes involved. Its essentially a reverse trannification in that they shove a bunch of funny things into his body instead of removing them and then feed the kid a ton of growth hormones and other funny things to make sure he grows up big and strong, like ceramic in his diet for his ossmodula to process to aid in bone density and strength. The implants don't even touch the bone marrow, instead the blood changes are done by the haemastamen and Larraman's gland.
Hell, IIRC Space Marines are even
ahem "proportional" where it really matters. There's a reason a Space Marine is more likely to look like a gigachad than a human being after all.