Friend,
You may not be able to convince them that is ok, hopefully you will be able to atleast have them "look over it" or have it be something they just don't think about.
You have to understand that your parents have aspirations for you and part of parenthood is dreaming and hoping and working your best to see your children become who you'd like them to be. It isn't becuase they are selfish, it is becuase they can't help valuing the things they do, so they can't help hoping you become something great in life, whatever their values are. Them pressing their own values on you isn't really a way to control you, its more them just being who they really are becuase you can't easily change your values you know? And they want the best for you... they can't really change what it is they value.
They want you to be strong, virtuous, noble, competent, have a family of your own one day... and to some people the diaper fetish is alot like homosexuality, most of the world hasn't been exposed to or cultured to the diaper fetish the way you have.
Your parent's are acting out of anger, but it can be its just because they are afraid you wont do great things in life you know? You can't really just come out with things like "Hi, i love diapers" most people don't even know what a diaperlover is, just that babies wear diapers and adults don't....
They can't help getting a little shocked and wonder what happened to their child and try to get you some help...
Just understand them and hopefully find a way to let it be something you simply don't discuss. They could possibly come to an open relationship with you where they accept it, but the world hasn't accepted us yet, it scares your parents too.