Why do teens get pregnant ?

Nope. Just sometimes immature with a false sense of the consequences of real life as well as still developing a sense of responsibility. 

yeah.  those boys who cant keep their nut in their balls are 
hella stupid. have some self control and pull out. 

Very perceptive.
My sister got pregnant at 13, deliberately. My mother forced her to get an abortion.
My sister then had 3 more pregnancies (with the same result.)
I asked her why. She said she wanted someone (the baby) to love and be loved by. I believed her.
At 17 she left home, got pregnant to a man who was married with 7 children and who made it clear that he would not accept responsibility for the child.
My sister proved a poor mother. She took the baby to India without vaccinations. The baby girl nearly died of some disease and had to be airlifted back home to a hospital, where it took six months before she fully recovered.
My sister regularly left vitamin pills and razor blades within easy reach of the toddler.
At 2, Sarah drowned. All the conditions were ripe to make it a highly probable accident.
My sister, Nikki, was packing to leave after a visit to Mum's place.
Sarah wanted to dress up in my sister's full-length nighty, and Nikki let her, thinking it would keep the child busy and out of her hair.
The door to the courtyard and pond was open to make it easy to carry out the luggage.
My mother was still in bed reading. I was 32 kilometres away in my studio in the inner city.
My mother had previously taken the protective grill off the fish pond "to make it more pretty."
Both Nikki and my mother knew that Sarah had a fascination with playing with the fish in the pond.
They also knew that the 9-month-old Doberman pup became very excited whenever Sarah went out to play with the fish.
Sarah went out the open door in the long nightie to play with the fish.
The pup became excited and knocked the toddler into the fish pond.
She became entangled in the weeds and drowned.
My sister never had another child.
My mother never wanted grandchildren.
She had discovered that she hated being a mother and did not want to be involved with children again.

Returning to the start, it was due to feeling unloved that my sister had wanted a baby.
This is far from the only reason why teens fall pregnant, but neither is it unusual.

What an unnatural abomination. 

Say it isn't so. 

Is so.

We must do what needs to be done.

Thank you for expanding my answer. You are much more poetic.