Complex sentence: This was a loveless union, and was presumaby a political accommodation: that such arrangements were possible implies the existence at that time of more Gondor-friendly Black Númenóreans than the much later Mouth Of Sauron.
Split into simpler sentences: This was a loveless union, and was presumably a political accommodation. That such arrangements were possible implies the existence at that time of more Gondor-friendly Black Númenóreans than the much later Mouth Of Sauron.