Why did those Mormon missionaries get defensive when questioned about the Principle?

Some Mormons came by my cousin's house today. They were there a few days ago as well, but this was their follow-up. He said they were polite and eager to answer all of his questions until he asked about the principle of plural marriage (I told him about it when I was there that first day). They then got "really defensive" and denied that the LDS Church ever had anything to do with it, then started demanding to know where he heard about it and left.