Well, I would take a moment and think about a few things. Would your girlfriend be less opposed if they were the small, "neater" dreads? Do you have very curly hair? Are you young yet? Are you male, and would it look ok if you shaved your head when you're over it? All questions I asked myself when I was considering dreads. I didn't opt for it, because I would look terrible with a shaved head, my hair isn't curly (I'd have to use chemicals), and it just wasn't me. Here I sit with crazy dyed hair, however, because I didn't go though that phase when I was young. If you really are in love with the idea, try it and get over the fascination. Neat little rows of dreadlocks are the only kind I like, if you want the female perspective, and do a little research or go pro with a hairdresser.
It's your hair, your life. Do not change who you are or who you want to be for anyone. If she truly lives you she will accept you no matter what. Plus your girlfriend today may not be your girl in your future. People change,people evolve, what they want today may not necessarily what they'll want in their future. It is not worth it to have regrets in your life. Shoulda, woulda,coulda...too late..