@thewizardguy And yet Space Corgis I'm more well read than you might think. I know that bit of canon and I accept that the Knights Eternal kind of side step around that but I can offer an explanation:
For the Knights Eternal, I'm going to chalk it up to a mix of slightly altered geneseed and the fact that they can't properly train and raise Space Marines since they don't exactly have all the medical tools or hypno/chemical therapy to do so; they got it to an acceptable level but there are a few small patches that they can't quite cover and use some other method to try and fill in the gaps. A combination of long term isolation, possible exposure to warp and lack of proper equipment can easily have small but noticeable effects I'd say.
They also don't actually have sex to have children; them and their wives simple have an artificial insemination where a sperm cell or some other cell that that can be used/be reversed into a stem cell, mingle it with an egg cell in a peteri dish or the 40k equivalent, and implant it into the wife. Boom, pregnant waifu without sex. The relationship also doesn't have to be romantic or sexual and can simply come out of serving along side one another and admiring each other's skill at arms or just enjoying company. Children that are "born" to such couples are also semi-raised as future generations of Knights/War Maidens since its assumed that their genes make them more inclined to such elite roles and in the case of the boys there's the hope that they will more easily adapt to the surgeries and the training to become an Astartes. So there is a more pragmatic side to child-rearing than simply hot Astarte-on-Sister action and fluffy fanfic romance.
And yes, there's still the chance of it going romantic and over attachment and blah blah blah but again, training and conditioning can work wonders. Plus there's always knowledge that the person may not be able to be with their spouse together forever since space marines generally live far longer lives than even rejuvenated women so there is definitely knowledge that one day someone is going to out live the other one.
I concede that this might just be an extension born out of my own personal interpretations of Space Marines which sees them less as ultimate warriors and angles of doom, death and destruction but super human guardians of mankind. They were created to protect humans so that's what the Knights Eternal ended up doing and I guess I decided I wanted to humanize them more whoops \_(ツ)_/¯
Also, this is 40k we are talking about, this is by far a far tamer creation/anomaly than some other things. Plus aren't you playing a xenos species that basically ignores the dangers of the warp while also travels faster that's not even in the canon? I know that you can argue that make a new species (ignoring what their FTL travel that ignores the Warp grants them) isn't too much of a stretch and I'd argue neither is me stretching what Space Marines are while also giving what I consider a decently sound argument and explanation. Plus, it
was accepted by the GM and I know for a fact that Manatee knows the same bit of lore surrounding the SM on this subject. Something else, don't need to act so passive aggressive or condescending mate.
Please do correct me if I'm wrong and have committed some rage inducing, unending hatred and heresy with irreversible consequences that absolutely ruins everything about the RP and 40k in general.
@agentmanatee can also join in on this if they want to as can everyone else in the RP.