It is a side effect of the Veiled One transformation into Daemons.
In ordinary parthenogenesis, the offspring are genetically so aesthetically almost identical. From the variety within Gehennians, it can be seen this is not the mechanism.
During Gehennian asexual reproduction, the genome (which is extremely large, due to the therapy and additions) has some areas shift or certain unexpressed areas come to the fore (such as being female). The formulae and augmentative substances contained within Gehennians, to allow them to become the enhanced beings they are, also effect certain genes or organisations differently (and are also affected by the personality as the Gehennian grows)
If two Gehennians reproduce sexually, as Umbran and Death's Shadow are rumoured to have done, the genes that come together allow far more powerful expressions of traits, as does the greater quantity of augmentative substances. As the mental 'destabilisers' can only affect one version of the gene, Gehennians produced sexually are far more mentally stable, and feel emotions alien to those of most Gehennians.
Due to this, Hectocapitus forbids Gehennian sexual reproduction.