Logic. She was using logic to win an argument against him. If the situation wasn't so dire and his own emotions weren't so volatile at the moment, he would have commended her on her decision. Then again, if the situation wasn't so dire and his own emotions weren't so volatile, he likely would have realized on his own that what she said was true and therefore he wouldn't be in this situation at the moment. It was ironic, really, that it had taken a human woman to make him see that he was the one being quite illogical and allowing his fears to determine his actions.
Of course, realizing all of that didn't make it any easier for him. He was still worried he would hurt her and wanted to argue that there obviously could be a great many reasons that his mother hadn't been harmed during his father's pon farr. Medically speaking, there were far more advancements in their time than in this one, and that was simply one possibility.
Yet Spock knew that none of that mattered. Nyota was right. He wasn't thinking clearly, had said as much himself mere moments ago, and therefore shouldn't be the only one allowed to have a say-so in this situation. It was an embarrassing thought but that didn't make it any less true. So it was with a soft tone and barely audible sigh that he admitted, "Logically speaking, my fears have no basis."
His own hand slid up, pressing lightly upon the one that she held to his cheek. "However I can only assume that my mother was well aware what she was facing," he added pointedly. "You do not and I am... incapable of explaining it, currently." He was still worried about her, about how she would handle his loss of control and a great many other things that went with it. He wasn't able to say it in so many words, but he was still quite worried.