Birth Control

Mariella • I'm hungry
So I'm 17 and I recently started taking birth control to try and minimize my crippling menstral cramps (literally I passed out one day in the middle of chemistry because of them). Although I'm not apposef to being sexually active I'm not currently and haven't been for almost a year. I was talking to some friends about it and one of them (she's super duper Christian) got super offended and told me that it was a sin? Wat? 
Also, last year my sister did a big project on who she thought was the most influential person in history and she chose the incenter of birth control (the pill). Her teacher said it was cool and so she did her huge massive project and made it to the "finals" of the project which was a debate between her and the other finalists, it was going to be judged by some local community members. Everyone made their opening statements and introduced who they were and what they did. Right after my sister made her speech the judges started talking and voted to disqualify her. 
That effected her grade.
What exactly is the issue with birth control??