The progesterone IUD (marketed as Mirena where I live) is much easier than sterilization and has a lower failure rate (even sterilization isn't perfect.) All the paranoid sex education we get makes it seem like pregnancy is almost certain if you're having intercourse. But there are excellent methods of birth control out there. For example if your GF goes on the pill (or patch or ring) and you use condoms the risk of pregnancy approaches zero if you're both using those methods correctly.
Have fun. You sound organized and responsible. I think if you're conscientious about contraception you're extremely unlikely to get pregnant.