4. It sends the message that marriage isn't important to you
I know that most people don't intend to send this message when they move in with their significant others; as I said above, many people I know chose to live together first out of a desire to avoid divorce. However, the message that you and your boyfriend or girlfriend send to one another when you set up house before a wedding is that marriage isn't that important as to be worth waiting for. When you cohabitate, you're implicitly saying that your future marriage isn't valuable enough to be worth tough sacrifices -- and that sets a dangerous precedent for when you do take the next step in your relationship. Combine that with point #1 about drifting toward engagement by default, and it puts a crack in the foundation of your relationship that could take years to fix, if it doesn't spread and get worse over time.