Not knowing the entire situation makes that a difficult question to answer, but I can tell you this. It all depends on what your parents "habit" is. If it is something personal between them, that doesn't concern you, or your boyfriend, and is really none of your business, then you probably shouldn't tell him about it.
If it's something that embarrasses you, and he somehow finds out later, i'm sure he will understand your reasons for not telling him about it. And i'm sure he will not hate you for not telling him.
If it's something concerning you, that could effect your marriage, tell him.
Like say, if he has a real problem with people that eat broccoli, and wouldn't want to be involved with a family that eats it, and your parents eat it, then maybe you should tell him.
On the other hand, if you know that your parents are sensitive about their broccoli intake, and would be upset if you told him, then you should keep their secret.
If you eat brocolli too... You should tell him.
This is a terrible example, I know. But I can only assume that in the back of my mind, I can figure out what your parent's "habit" is. So I figured this example would serve it's purpose. I could be wrong however, there are a million things it could be.
At any rate, if you don't want to post it publicly, but you need someone to talk to, you can send me a pm anytime, and we can talk. If you want.
Confidentiality between your parents and you vs. Honesty with your fiance, is a hard road to travel. I'm sorry that your in this situation.
~qriosity~