What are your opinions on a couple living together before marriage? Future mother-in-law doesn't want us to?

My boyfriend and I want to move in together in about 1-2 years. We plan on marriage in 3-5. My future mother in law happened to hear us talking about it one day, and said she would be very unhappy with us if we moved in together before we were married, that she didn't want us doing it, using that stern "I'm the mother and you will listen to me" voice. Now, I am aware that, having an adult relationship, we make our own decisions without the approval of others. However, I would like to do it without making my future mother in law hate me. How do I do that, and what are your personal opinions on a couple living together before marriage?