I think it is a very important part of a relationship. It is a form of trust that a couple have with each other. That's why we call it "Making Love" right? It should happen when the love and passion accelerate to a point that you just want to give yourself to each other and be completely naked for each other....both physiclaly and emotionally.
I think sex outweights love more in the beginning of the relationship. But as time goes by, love will play a bigger roles in maintaining the relationship. Sex is important in all relationship.