Both Dr. Ferguson and Mabel, following his rescue of her from trying to drown herself, experience a serious sexual awakening as they literally are overcome by passion and their reason is overpowered by feeling. What is important to note is that we are told this scene in the story focusing on the feelings of Dr. Ferguson rather than Mabel, so we can only see and hear what Dr. Ferguson is thinking and just hear the words and see the actions that Mabel does. Dr. Ferguson feels in Mabel's control and overcome by desire:
He very much wanted to go upstairs to get into dry clothign. but there was another desire in him. And she seemed to hold him. His will seemed to have gone to sleep, and left him, standing there slack before her.
The story continues by saying "It was as if she had the life of his body in her hands, and he could not extricate himself." It is clear that Mabel herself is undergoing the same realisation of sudden, passionate and overwhelming love, as is shown by her actions:
She shuffled forward on her knees, and put her arms round him, round his legs, as he stood there, pressing her breasts against his knees and thighs, clutching him with strange, convulsive certainty, pressing his thigs against her, drawing him to her face, her throat, as she looked up at him with flaring, humble eyes of transfiguration, triumphant in first possession.
Both then experience a sudden sexual awakening of interest and love in the other that overcomes their reason and other doubts that they have, such as the doctor's concerns about what others will think about him.