Just a hypothesis I guess. But what if the man, feeling her distancing and through her micro changes to her patterns, he starts getting competition anxiety. He feels or knows the inevitable is closing in. In an effort to appease or ward off her natural propensities by propitiation, he brings it up. The woman understanding her condition at, at least some level, manipulates this condition to her favor or advantage. This would, for at least a while, ward off the biological urges that she doesn't have the capacity to fight.
She gets sperm competition for her ovaries, (Sperm Wars, by Evolutionary Biologist, Robin Baker & The Red Queen by, Matt Ridley), however his thinking is flawed in that it lowers his value to share her reproductive system with other men. In her eyes. (Hind Brain)
It robs him of his offspring, potentially, and thus is not in accordance with his biological imperative.