If you are a minor (under eighteen years), and you live in the United States, you have rights to confidential reproductive health care. What this means is that unless you consent or give permission to your health care provider, s/he cannot disclose your medical records to anyone, including your parents (except in the case of abortion services, which depends on your particular state's law). Additionally, since no state or federal laws exist at the present time that would prevent minors from obtaining contraception, teens don't need parental consent or notification to get birth control pills, condoms, emergency contraception, and other contraceptive choices. According to The Center for Reproductive Rights, "Access to contraceptive services is considered a fundamental privacy right and has remained so for over three decades."
I am pretty sure you can go to planned parenthood and they will get you the pill without your parents. However, if she has given you sex before, she probably won't last long.
You may need to go to a physician to get the pill if planned parenthood doesn't have a service. If thats the case just pay in cash and it should all be confidential. Your tracks will be covered
Btw your not an ass for wanting sex. Your right if the only thing you wanted was sex, then you could be considered an ass. Don't be afraid to tell her that its an activity that you want to pursue before marriage and that you want to do it right. Which means getting the right kind of contraception.