Is Marriage Natural?

Scarlett Johansson, who was recently divorced for the second time, said in an interview: "I think the idea of marriage is very romantic; it's a beautiful idea, and the practice of it can be a very beautiful thing. I don't think it's natural to be a monogamous person. The fact that it is such work for so many people -- for everyone -- proves that it is not a natural thing." So is monogamy natural? If so, why does it take work? If not, how can we make marriage work?