This is similar to other models of courtship behavior. Essentially, females prefer males that invest in their relationship because they are more likely to remain in the relationship. This same phenomenon can be found in all sorts of things, such as law offices (why are they so fancy?!).

A great general model on this can be found here (pdf): http://octavia.zoology.washington.edu/publications/forthcomi...

This is a chapter from a book (P. Zak, ed. Moral Markets: The Critical Role of Values in the Economy. Princeton University Press).