Well sounds like you already know they're gonna flip out... Hmmm... Wonder why??? Oh , I know... You're too young. You haven't finished high school , much less college. Here's an idea. Don't do it. If he loves you at 17 and it's really gonna work, then he'll still love you five years later. If you've gotta be married for him to love you, then it's not true love anyway!
At 17, your parents still see you as a child and they might be against it. The only thing you can do is talk to them like an adult and prove that you are mature enough to make wise decisions. It might take sometime for them to accept your choice but you have to be patient and understand that they want the best for you.
Three of my daughters married at 17, my maternal grandparents were 16 & 17 when they married, had a sister marry at age 29 and her groom was 30, it was a first marriage for both. Age really doesn't matter much. I think it's more important to commit to one person for life as your lover than it is to try to please society/parents by waiting for some arbitrary age to commit. Been married to the same woman for 34 years and i encourage you to do what you know is right, don't let anyone else do your thinking for you.
Age doesn't matter in love? Unfortunately being able to take care of yourself does. School, college, grad school, work, job, career, car, car insurance, health insurance, house, apartment, baby........... At 17 you may be in love, but you have a tuff road that people twice your age struggle with. I quess you could say you are eternally engaged for now, with possibility of marriage someday. That may ease her mind. Concentrate on the priorities that are needed to have a good life.