If you plan to honor each other (offer and have accepted your last name, agree to sleep with no others, etc) and honor any children your union produces ( by your love, support, last name, insurance coverage,etc) then marriage would be putting your money where your mouth is; otherwise, just sleep around, have others raise (your?) kids in a single-parent household or worse, keep your money and be free! How great for you!
No, you don't need to be married to spend the rest of your life with someone. It's sad that most people don't respect people who choose to be with each other and not marry. They are not considered family, or a real relationship. If you want to be with someone for the rest of your life without the marriage then you go for it.
They do not have to get a legal civil marriage, but if they are in a loving and committed relationship--a relationship otherwise defined as marriage--they will not be afforded the rights, responsibilities, and privileges extended to legally married individuals no matter where they go throughout the world.