My gf doesn't wanna have sex with me because..?

We've known eachother for 5 years and we've been dating for two months. We both think we are perfect for eachother there's no one that can take the place of either one of us. We even promised eachother to marry after college, also talked about having a kid. When we talked about sex(she's a virgin btw), she said she'd like to wait until the day we marry. My jaw literally dropped on the floor because that's like 5 years of literal torture! When I asked her why wait that much, she said she's afraid I'll grow tired of her and dump her, after having sex. She even suggested me to sleep with other people just not to lose me! Then I think I calmed her down, she said let time decide.. Now there's an awkward tension that I really hate, I don't know what to do. Any help is appreciated.