I think that Amy L makes a very good point--THE POINT--really.  If we love Jesus, we want to please him.  If we fail in this mission, if we commit sin, we believe in his power to forgive us.  This is the message I want to give my kids:  Jesus has no use for perfect people.  He knows we will sin and that's why he came.  But we don't get around guilt or shame by redefining sin--and premarital sex is clearly defined in the Gospel as a sin--not THE preeminent sin, not the worst sin we could ever commit, but a sin, which is worthy of shame. Shame at the act of extramarital sex is pretty much a universal, which leads me to believe that it is not Churches which instill this feeling in us, or parents, or other Christians.  If it were not written in the Gospel, it would still be written in our hearts, because so many people who are not Christians still have shame at the experience.

I tend to think of it more in terms of what we can learn from our guilt and shame.  Like you, I was once in a relationship where I thought that we were too young and healthy and in love to NOT have sex.  I dated that boy for three years, became engaged to him, and finally, FINALLY realized that a lof of that guilt and shame and incredible sex was distracting me from the fact that I was about to marry the WRONG GUY.  Nevertheless, I made the exact same mistake shortly thereafter with another wrong guy.  Sex is good.  It is powerful.  And by the time I met my husband, we had both been around the block enough to know that we didn't want to screw up our relationship with the fog of pre-marital sex.  As a result, I have the distinction of being the only girl he ever dated seriously who he didn't screw, and vice versa.  But the clencher, the key, was that to NOT have sex before marriage, we both had to entrench ourselves in prayer and Sacrament.  We could not have survived chaste courtship (especially as we'd both been sexually active in the past) without a mutual desire to make our relationship pleasing to God.