Before you get married is too soon. After the marriage is perfect! If you simply live with a guy for a while, then move on to someone else, when you finally do get married, it is much, much easier for you to walk away when the relationship hits a rough spot rather than stick it out.
Make your relationships special - but always understanding that when it comes to marriage, you truly want to make a permanent commitment no matter what. Ifyou're not ready to do that yet, you aren't ready to move in with your boyfriend, either.