Why does fanny think that it is bad that lenina has been going out with no one but henry foster for four months?

Lenina Crowne, a Beta Plus, talks about her four-month association with Alpha Henry Foster with her companion Fanny Crowne, a Beta. Fanny is disturbed that Lenina is having such an involved acquaintance with just a single man. She cites the expression "everybody has a place with everybody" and advises Lenina to engage in sexual relations with other men. Lenina concurs with Fanny and discloses to her that she loves Bernard Marx, an Alpha Plus, and has chosen to go along with him on an outing to the Savage Reservations. Fanny is suspicious and says that she supposes Marx is a recluse and a self observer.