It sounds to me like you have found yourself in a tough situation. First of all, did you know that your parents were so much against the idea of premarital sex? If in fact you did know, had you discussed your opinions about it with them?
I think that it is important to realize that regardless of their views on sex, it is always difficult for parents to find out that their children are sexually active. In addition, I think it is also important that you realize that you are quite young which gives them even more reason to be worried.
Protect Yourself With the Pill and Condoms
I also want to express a concern of my own. While I realize that you are in love and you are happy with your decisions, please be careful. In this day and age, it is SO important to protect yourself from sexually transmitted diseases and pregnancy. Please make sure that you put yourself on the pill and that you are using a condom. This will protect you from both pregnancy and STDs.
Your Parents Won't See Your Side Of This
It is going to be difficult to get your parents to see this situation from your point of view. In fact I would say that the chances are almost nil.
Parents Are The Best Advice Givers
It is most important that you give them time to think about this. In addition, I think that it is of utmost importance that you discuss this issue further with your parents. You will come to realize as you grow older that your parents are the best advice givers and sounding boards because they probably know you better and have known you for longer than anyone else.
Find Out Their Beliefs
It is good for you to begin making decisions on your own but remember that your parents have years of experience which they are coming from and this is probably reflected in their opinions and beliefs.
You may want to hear them out just as they should give you a chance to make your case. Don't push things with them just yet- this is a very difficult thin for parents to come to terms with and you need to give them time.
Cool Things Off With Your Boyfriend
If your boyfriend loves you as much as you love him, he will
be patient and understanding and maybe you two should cool things down a little while your parents have a chance to swallow things and you have a chance to discuss things with them.
Maybe the answer is to show them that you are mature and able to make these big decisions.
Finally, While I understand that your boyfriend makes you happy, I am also concerned about his past. Does he have a sexual history? If so, please send him to get tested.
My advice in a nutshell? Be patient, explain your side to your parents and make sure you give them a chance to be heard too,and hang in there. Things will eventually work themselves out.