Yes they are able to make that decision. You are only 17 years old and it is VERY possible that years from now you will want more children (even though I'm sure it is not something you want to here now) Let's just say this marriage does not work out (fingers crossed that it will) and you you marry again 10 years from now and he wants children? Or what if you do change your mind? Or better yet what if the fiance you have now changes his mind? There are so many "what if's" that you are facing because of your age and situation that it probably would be best for you to wait to have your tubes tied. It would be very different if you were older and had more children. Another reason may be that you are not at the legal age of consent.....