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, discusses her four-month relationship with Alpha Henry Foster with her friend Fanny Crowne, a Beta. Fanny is upset that Lenina is having such a long relationship with only one man. She quotes the phrase "everyone belongs to everyone" and tells Lenina to have sex with other men. Lenina agrees with Fanny and tells her that she likes Bernard Marx, an Alpha Plus, and has decided to join him on a trip to the Savage Reservations. Fanny is skeptical and says that she thinks Marx is a loner and an introvert.