Carly,
Congratulations! I'm glad to hear that your boyfriend is being supportive, caring and responsible for the pregnancy. Raising children within a loving marriage with a father and mother is obviously the ideal situation. I think that it will be good for your baby's father to be in your life and in your child's life. However, marriage is wonderful, but sometimes difficult. If you aren't sure you love him or have doubts about marriage don't rush the decision because of the pregnancy. Marriage is not a decision to be taken lightly.
You both sound like you are taking a mature, responsible approach to the situation. You're relatives may be upset and dissapointed at first, but I bet they will come around after a little while and be supportive of you. I'm not sure how to tell them. When I was younger and had a couple of pregnancy scares I dreaded the thought of having to tell my family. This time both my family and my husband's family were practically begging for grandchildren so it wasn't difficult to give them the news. Would it help for your boyfriend to be with you to break the news about your pregnancy and engagement to your relatives? I would probably set down together and share the news. At least he could be there for support if things get rough.
Kate