Ive heard of Drs doing this... it is because you are young and they are worried about your well being in the future if you regret doing it so early... so health wise there is the mental concern... i would approach the subject again tho and maybe the Dr will see that you have thought this through, know your subject and show that you have done the research...
I understand why some are cautious about the subject but its really up to you in the end and its too bad that you are being boxed in with others especially if this is what you want...
I'm shocked he would deny you. Yes you are young, but if you have a few children (which you obviously do), most doctors won't say no. heck I'm 26 with my second on the way and my ob asks me almost everytime if I want my tubes tied or not >:-(.
I don't know why your doctor won't, I'm 22 and pregnant with our third and my doctor doesn't have a problem with tieing my tubes. Maybe you should try talking to him again and if that doesn't work switch to a doctor that will.
There should be a form that your husband can sign to allow you to get the procedure. Im 20 and pregnant with my first and i am allowed to get my tubes tied after i give birth. Just be sure this is what you want though.