Co-habitation can be seen as a replacement for marriage. It has all the same roles and meanings, two people living together probably working together to pay the bills and be able to spend their life together. It just isn't made official or has any papers to back up their commitment.
Although co-habitation is becoming more popular in the modern world, it will never replace marriage. People still seek the commitment of marriage for a sense of security. Also, a lot of gays today are demanding the right to legally marry. Clearly, the institution of marriage is not going anywhere any time soon.