Catholic girlfriend wants ?

Ive been going with my girl since we where 14, we are now 18 and she says that shes waited long enough for and now that we are legal we should do it. Problem is Im Evangelical and I want to wait until Im married to have , partly because that's what I believe and my folks would kill, but she says Im being selfish and that she has needs, I don't know what to do Im scared she will leave me if I don't, but I don't want to go against my church. What should I do?