Marriage....

Hey everyone! This is the first time I have ever posted here so bear with me. I have been with my boyfriend for 6.5 years. We have 2 beautiful kids together and plan to have more in the future. There is nothing I would love more than to get married to him! I always try to bring it up and he use to tell me to just be patient. My patience has run thin. It went from just be patient to I don't know. I finally got a why out of him and he said he has heard from everyone at work that marriage changes everything and he doesn't want that to happen. Marriage means a lot to me and it's something I want. I am just tired of waiting around for something to happen when it probably never will. Any advice as to what I should do?