When your girlfriend suggested a two girls one guy threesome, you weren't against it.
She told you to undress on the bed and wait for her and her friend.
When her and a naked dude who was taller and more muscular than you entered the room, you were absolutely shocked.
You stood up and shouted at her to explain it.
When the guy went over to you and you grabbed his hand, suddenly all your strenght left you.
He pressed your back onto him, lifted your legs up and put his dick between your legs.
When he thrust his dick into your ass you felt even weaker. Your dick got hard, but it was growing smaller. It was sensation you never felt, it hurt, but you came anyway.
Your body was changing, with each and every thrust. You were getting smaller, your skin more soft. Your moans were higher and higher pitched. Little mounds started forming on your chest. Your hips were getting wider and wider. Your hair was growing rapidly. Suddenly your penis was no more and you had a pussy.
When he took out his penis and rammed it right into your newly formed pussy, the feeling got many times better. It hurt for a second, but then it felt very smooth as you were plenty wet already. The changes were happening now even faster.
You tried to think of something, but it was getting harder and harder to think. You begged him to stop, but he kept going. You were closer and closer to cumming, until you came hard on his dick, and then he came hard into your gushing pussy.
When it was all over you were panting from exhaustion, you thought, is this my life now?
Your girlfriend finally joined in and started fondling and kissing you. "Welcome to womanhood" she said.