As much as this sucks to say: They are your parents, and they have the right not to let you play something.
Best thing you could either do is wait till you're 17, or, sit down and have a chit-chat with them about why you feel you should be allowed to play them. But, if they still say no, then try to respect their decision. If anything M rated games usually expose you to material that isn't the greatest. So eh, they're the ones in charge, and you have to listen to what they say.