This is wikipedia/Human_sexual_response_cycle , converted into an erotic short story by pruning some less sexy parts, approximately changing ‘male’ to ‘his’, ‘female’ to ‘her’, and changing from present to past tense.
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Their sexual response cycle
Their response cycle was a four-stage physiological response to sexual stimulation, which, in order of their occurrence, were the excitement phase, plateau phase, orgasmic phase, and resolution phase. Their cycle was first described by William H. Masters and Virginia E. Johnson in their 1966 erotic novel Their Sexual Response. Since then, other sexual responses have been experienced.
The excitement phase was the first stage of their sexual response cycle. It occured as the result of physical or mental erotic stimuli, including kissing, petting, and viewing erotic images, which lead to sexual arousal. During the excitement stage, their bodies prepared for sexual intercourse, initially leading to their plateau phase. Physical and emotional interaction and stimulation of their erogenous zones occured during foreplay, establishing initial arousal.