शक्तप्रजननयोरप्युपस्थितप्रजननतो विशेषः ॥ कZ_०७.१७.३० ॥
śakta-prajananayoḥ apy upasthita-prajananato viśeṣaḥ
Even when both are capable of procreation, the differentiator is the presence of already-existing offspring (i.e., heirs already produced).
It reduces dependence on the pledged heir, weakening the constraint and increasing the likelihood of opportunistic treaty breach.