You didn't care and took her anyway.

You turned and stepped in front of her, slowly taking in her body with your eyes. she looked at your face, then her eyes lowered to your stiff cock.
"You like what you see, don't you?" you asked and began to trace a fingertip over her chest, between her breasts and rested just above her navel. Your gaze moved from her trimmed patch up to her face and she nodded. You looked sternly at her, "Rub yourself, Lisa."
She leaned back supporting herself with one arm, while she reached between her legs with the other. She slipped her finger between her folds and dragged it up to her clit. She began to rub it in circles and moaned quickly after. You nodded in approval and held your cock in your hand.
"Quicker," you commanded, "but do not come!"
She increased her pace and panted heavily between moans. Her arousal climbed quickly and she bit her lip to prevent herself from climaxing. her eyes started to flutter from the intense pleasure.
"Stop!" you said as you noticed how close she was, "Spread your legs for me, Lisa."
Reluctantly she lifted her finger and looked at you with lust in her eyes as she slowly spread her legs. You positioned yourself between them and placed the tip of your cock against her lips. Slowly you dragged it up and down, occasionally rubbing it on her clit. Moaning she started to tense and relax with your movements, at which point you pulled away.
"Lisa, what are you doing? I told you not to come and now you're working your way to it again. I wonder if I should punish you for that," you said shaking your head.
"I... I... It feels so good, I can't help myself, professor," she uttered pleadingly, "please don't punish me for that."
You nodded at her and began to slip your cock up and down between her lips, "Okay, I won't punish you yet, but this is the only warning you get. Next time..." Your voice trailed as she began to moan louder and you smiled.

You continued to rub your cock along her folds and every few rubs you pushed slightly into her, causing her to moan longingly. You examined her face as you pushed your glans inside her and held still. She moaned from the sensation, then her eyes widened with worry with the last straw of sensibility.
"Please... Don't, I don't want to get pregnant!" she begged you half pleading, half moaning.
"Don't what, Lisa?" you asked as you pushed a little deeper.
She moaned and shivered, "Ohhh.... That!"
You pulled out slightly. "Are you sure, Lisa? You'll miss out on the great pleasure of your climax," you insisted.
Her head moved up to start to nod as you pushed an inch inside her again and held still. You twitched your cock and looked sternly at her, "Well? You want my cock, don't you?" You pulled out again and pressed the tip between her folds, "Tell me you want it."
She lost it and moaned longingly while she closed her eyes nodding, "Yes, ... I want ... your cock."
That was all you needed and you shoved your full length inside her. She shuddered on your cock moaning loud. You pulled back and plunged in, building up a steady slow pace. She began to rock her hips to match your thrusts.
"Lisa!" you said holding still and she froze looking at you pleadingly.
"I'm sorry, I'm sorry. Don't punish me," she whimpered.
You narrowed your eyes and twitched your cock while demanding, "Beg for it!"
Her pussy spasmed in response to your demand and twitches and she begged, "Please don't stop, it feels so good. Please..."
"Hmmm, it'll have to do, I guess," you said and resumed your steady pace. You examined her face and body as she began to moan louder. You increased your pace and she began to shudder. Suddenly you thrust your throbbing cock hard into her and held still.
"Come now, Lisa," you commanded with your cock buried deep inside her, throbbing and pulsing.
Your words tipped her over the edge, she threw her head back and let go. Her pussy clenching and relaxing in waves on your cock as her hips shuddered. Wave after wave coursed through her, her body rising and falling. You pulled back a little then plunged in deep again, groaning as you released thick jets of cum deep inside her. Small thrusts followed, while her pussy milked you, keeping her in the throws of her climax. You even rubbed her clit and managed to take it even higher.
"Good girl," you murmured, "You're a very good girl. Say that you are my very good girl, Lisa."
She moaned gazing at you, "Thank you." She meant it, the climax was amazing. In her haze she admitted, "I'm your very good girl."

You smiled and then it all started to ebb. You held your cock still inside her as it grew limp and caressed her body.
Again you murmured, "Yes, my very good girl. You are going to come to me for another practical application after the next lecture." It wasn't a question, it was an order. She could only nod, still not fully back to her senses. You pulled out of her and picked up her panties to clean your cock, threw them at her, then pulled up your boxers and jeans and buttoned up.
"That was very good, Lisa, I'm certain that you're enjoying the release of endorphins intensely." You turned towards the door, "Make sure there are no stains on the desk or floor before you go, Lisa. See you at my next lecture." After that you stepped out of your office and pulled the door shut behind you.

She was stunned from your words and shocked at the realisation that you'd filled her with your seed. Tears rolled from her eyes as she started to dress herself. She had conflicting feelings, it was great, really great. But she also felt used, very used. Yet, somehow she liked that too. The only thing that worried her deeply was the risk of being pregnant from you.