Outside of the fact that she got pregnant, what makes you think that they were not using contraceptives? Birth control pills fail, condoms fail, all methods of birth control, even a vasectomy, have been known to fail. That said, the thing with teenagers and raging hormones is they may not have ever intended to have sex but got carried away in the heat of the moment. Happens all the time. It happened in the 1950s when I was in high school and I am sure it happens today.
It reminds me of an old Blondie cartoon. Dagwood's daughter is walking in all moon-eyed and rejects her parents advice to discuss the situation with a terse "Your generation doesn't understand love!"
Dagwood turns to Blondie and remarks "I suppose she thinks we're just good friends."
And how was it looked on back in the 1950s when you were in high school?