Marriage Inside the Church Building

Could you please explain the reason that for a sacramental marriage to be valid it must be performed inside the church building? I'm not a thealogian by any means but i thought the people were the church. I don't know all the rules by in many cases a mass can be celebrated outside but a Catholic wedding can't be. I've been told it has something to do with the sanctity of the church ground. Please clarify this issue for me and thank you very much.