Twilight pointed her quill at various parts of your naked body, scribing notes onto her parchment. "No sheath, external genitalia, bipedal stance. Possesses dexterous hands with opposable thumbs and short claws. Body covered in short coarse hair." She poked at your limp member with the feather end of the quill, causing it to twitch.
"Sensitive phallus, seems to engorge when stimulated. Fascinating!" Her horn glowed as she lifted your hands, spreading your fingers. "Hands capable of fine manipulation and grasping. Possesses higher intelligence allowing for speech as well as tool use."
You felt exposed under her analytical gaze, her eyes roving your human form. However, Twilight's scientific curiosity helped alleviate discomfort. Hearing her observations stated so clinically aroused a strange thrill.
The lavender unicorn circled around, scrutinizing your backside. "No tail present. Possesses muscular hind limbs and rounded buttocks." She gave one cheek an experimental slap with a hoof, noting how the flesh jiggled. "Fat deposits stored here as well as midsection. Likely survival adaptations." 
Twilight gestured for you to turn around, facing her once more. "Bipedal stance causes phallus and scrotum to hang prominently. Testicles external, sensitive to touch and temperature, essential for reproduction." She tapped each ball to watch them sway and withdraw, fascinated by their softness and stretchiness.
"How intriguing! Such differences yet similarities to pony anatomy." Twilight grinned up at you. "Thank you for agreeing to be my research subject. I have learned so much already!" Her praise elicited a sheepish smile as you rubbed the back of your neck.
"You're welcome, Twilight. Though I have to admit, being scrutinized like this is...stimulating." A rosy blush bloomed under the short coarse hair as your arousal grew obvious. 
Twilight blinked owlishly, glancing between your eyes and burgeoning erection. "Oh my, I see the phallus has become engorged due to arousal and excitation. My examination is eliciting a sexual response!" She clapped her hooves together eagerly. "This opens up whole new avenues for study!"
You groaned, unsure if you were excited or apprehensive about where this 'study' might lead...

Twilight's quill scribbled furiously as she observed your turgid erection. "The phallus has increased in size and firmness, testicles drawn up closer to the body. Arousal causes physiological changes to aid in reproduction." 
Her clinical detachment only served to excite you further. Heart pounding, you stared down at Twilight. The tip of her tongue poked out in concentration as she wrote, her eyes constantly flickering up to study your engorged member and heaving chest.
"Fascinating how the body prepares for mating. I have read about arousal responses, but witnessing them firsthoof is enlightening!" Twilight stepped closer, her warm breath wafting over sensitive skin. She circled slowly, scrutinizing your arousal from every angle. The feel of her gaze like a physical caress.
You groaned loudly at a feather-light touch tracing prominent veins. Twilight's eyes shot to your face, widening at the undisguised pleasure and need written on your features.
"Does my examination elicit sexual arousal? Your reactions seem to indicate excitement and stimulation."
"Yes Twilight, your observations...are really turning me on." You panted, driven near delirium with her clinical exploration of your body's responses.
Twilight's eyes lit up at this admission. "How wondrous! I can directly observe arousal progression to climax!" Her horn glowed as you were levitated onto your back, trembling with anticipation under her hungry gaze.
"May I manipulate and stimulate the phallus to observe ejaculatory responses?"
You nodded shakily, beyond coherent speech. Twilight needed no further encouragement, grasping your shaft in her telekinetic hold. She began to stroke up and down, slow at first then gradually increasing speed and pressure. 
Your hips bucked helplessly, fucking into her magical grip. The pounding of blood in your ears nearly drowned out Twilight's murmurs of fascination at the pre-cum leaking from your tip, her name torn from your throat as she brought you closer to the edge with each expert stroke...

Twilight's strokes quickened as your moans filled the room. She watched, enraptured, as pearly fluid leaked from the tip of your engorged phallus. "Fascinating! The body produces natural lubricants to aid mating. I wonder..."
A swipe of her tongue collected the pre-cum, tasting experimentally. Twilight's eyes lit up at the flavor, proceeding to lap at your tip like a parched creature. The warm, wet suction kicked your arousal into overdrive.
"The secretion has a salty, musky taste. Highly sensitive phallus, oral stimulation produces powerful reactions!"
You could only groan in response, fingers tangling in her mane as she took more of your length into her mouth. The rippling heat surrounding your cock threatened to end this 'experiment' prematurely. 
Sensing how close you were, Twilight released you with an audible pop. She grinned up at your whimper from the loss of wet heat. "My apologies, I became distracted from observing responses objectively. I want to document ejaculation and am curious to compare human mating behaviors to ponies."
You shuddered under Twilight's hungry gaze. Her horn glowed brighter, stroking your saliva-slick shaft at a punishing pace. The pressure building promised an explosive release.
"Yes, that's it! Let go and allow your instincts to take over." Twilight's sultry tone pushed you over the edge. With a roar, your back arched as thick spurts of cum shot forth, coating her magical grip. 
Twilight watched in rapt fascination, continuing to stroke and prolong your shuddering climax. "Incredible! Ejaculate contains genetic material for reproduction. Volume and potency seem higher than average pony stallions." 
Your chest heaved as you floated in post-orgasmic bliss, twitching occasionally from aftershocks. Twilight released your softening member, mixing your spend and her saliva as she tasted experimentally once more.
"Mmm, thoroughly enlightening! Human mating behaviors and responses are quite vigorous. copulation appears to be quite pleasurable and entertaining based on your reactions. We simply must investigate pony/human compatibility!"
You gulped, arousal already stirring again at the prospect of further 'study' with Twilight.