Opinion on sex before marriage?

I was just wondering what your opinion on sex before marriage was. All throughout my childhood and teenage years all I heard my mom talk about was about how sex was for married people and how she would be so disappointed if one of her daughters was out having sex and still living under her roof. Even though she tried to raise me to believe sex is bad and that it's a sin that will be punished later in live, I have a healthy sex life with my boyfriend who I have been dating for over a year. We are both 19 years old, are in college, love each other, and are extremely responsible and careful when it comes to sex. In my opinion, I'm not doing anything bad and no matter what she says I will not change my mind. I do understand where she's coming from; I do not agree with promiscuity, but I don't see the harm in having sex if you are in a serious committed relationship whether you are married or not. I could NEVER tell her I'm having sex, but sometimes I just feel like blowing up on her and letting her know having sex before marriage doesn't mean you are down the wrong path. Opinions??