Describing a sex moan in writing requires careful consideration of context, character, and desired effect. There's no single "right" way, as the nuances of a moan are highly dependent on these factors. Here are several approaches, ranging from subtle suggestion to explicit detail:

Subtle Suggestion:

• Focus on the physicality: Instead of directly describing the sound, describe the accompanying physical actions and reactions. For example: "Her breath hitched, a tremor running through her body. Her fingers tightened around his." This leaves the sound implied, allowing the reader to fill in the blanks based on the context.
• Use evocative verbs and adjectives: Words like "gasped," "whimpered," "uttered," "strained," and "trembling" can suggest a moan without explicitly describing it. Adjectives like "low," "raspy," "throaty," or "soft" can hint at the quality of the sound. For example: "A low, throaty sound escaped her lips."
• Employ figurative language: Metaphors and similes can create a vivid image without being graphic. For example: "A sound like the sigh of a contented cat escaped her." or "Her moan was a wave, cresting and falling."

More Explicit Descriptions:

• Vary the intensity and pitch: A moan can start softly and build to a crescendo, or vice versa. Describe the changes in pitch and volume to convey the intensity of the experience. For example: "She began with a soft, drawn-out moan that gradually intensified, rising to a breathless cry before fading into a low hum."
• Use onomatopoeia sparingly and strategically: While onomatopoeia (words that imitate sounds) can be effective, overuse can feel cheap and cliché. Use it judiciously to enhance the description, not replace it. For example: "A low 'mmm' escaped her lips, deepening into a drawn-out 'ah.'" Avoid excessive repetition of simple sounds like "mmm" or "ahh."
• Consider the character's personality and emotional state: A shy character's moan will differ significantly from a confident one's. A moan of pleasure will sound different from a moan of pain or exertion. Incorporate these nuances into your description. For example: "Her moan was hesitant at first, a shy whisper, but as the passion intensified, it grew bolder, more confident."
• Describe the surrounding environment: The acoustics of the room can affect the sound of a moan. A small, enclosed space might amplify the sound, while a large, open space might muffle it. This detail can add realism and depth to your writing. For example: "Her moan echoed softly in the dimly lit room."
• Focus on the rhythm and phrasing: Moans are rarely monotone. They often have a rhythm and phrasing that reflects the intensity and ebb and flow of the experience. Try to capture this rhythm in your writing. For example: "The sound rose and fell, a rhythmic pulse that mirrored the beat of her heart."

Remember to always consider your target audience and the overall tone of your work. What might be appropriate in one context could be inappropriate in another. The key is to be evocative and suggestive, using language that creates a vivid image in the reader's mind without resorting to gratuitous detail. Balance explicitness with subtlety to achieve the desired effect.