I understand why they're upset. Your families are very close and basically considers you all family. The fact that youre dating can possibly ruin everything between your family. What happens if you break up and are on bad terms and don't want to see each other anymore? Realistically, you're both very young so the chances of this relationship lasting and ending in marriage/kids is slim; of course it doesn't mean that you'll break up but it would make things very awkward with your families and create a lot of conflics/drama. You have to keep in mind that your family considers your gf family as well; so if you do break up, your families will probably still be close and you'll see her very often even if she hurt you in some way or you hate her. Just something to consider since you want to keep dating each other.