Ok wow, first of all, If you are twenty years old, I think the whole "OMG PLEASE don't have sex. it will ruin your life!!
" bit should stop..in my honest opinion.
In other news, if the first doctor simply will not give you birth control because of a philosophical reason like that, and not a medical reason, then it's either time to go to a different doctor, or tell your doctor to be a doctor not a pastor. If you wanted advice on the ethics of premarital sex, you could have gone anywhere else to other people who are more qualified to answer that question.
As far as a doctor goes, their responiblity is about keeping your body healthy. And as far as I know, saying "I do" does not drastically change the effectiveness of birth control or the effect of sex. Heck, I know some doctors prescribe birth control to 13-14 year old girls to stabilize their periods. No one is assuming they are having sex. So, unless he has any good MEDICAL reason to deny you birth control, allergic reaction, hormonal imbalance, or something, he has no reason to deny you the medicine you request.
If all else fails, either go over is head and explain why he denied you the prescribetion you requested, or go to antoher doctor to have a second opinion.
"I am a doctor, not a pastor."