Why is it considered wrong if an unmarried couple decides to live together?

I love my girlfriend and I told her that I plan on marrying her. She is receptive to this and loves me dearly as well. My parents were not married yet their love for one another was strong until death parted them when I was a teenager. I am now 37, and having grown up in a household where I knew the importance of love, I value monogamy. Exclusivity, respect and faithfulness are some of the things my girlfriend and I prize in our premarital relationship. She said to me that she would be happy with me even if we weren't a married couple. I feel the same way with her. I would appreciate opinions and wholehearted responses to the aforementioned question about living together.