I think she's smart. Think about it for a moment...
Manson is old and in prison and won't be allowed any visitations. She will be married to him and thereby the legal heir to any money made off the Manson story after he dies. She won't have to touch the foul creature, but will reap the financial benefits when he dies. There will be movies, documentaries and books. She will get a piece of all of that. It's a good deal for her.
Then again, she might just be an idiot. There are a lot of those out here.