Why does Fanny try to convince Lenina to be more promiscuous in Brave new World?

"When the individual feels, the community
reels."

It's not much more complicated than that.  Sex
based on relationships can lead to all kinds of "bad" things ... frustruation, anxiety,
possessiveness --- all the things that we learn about in the text when it speaks of the
"family" as is used to exist.  Sex, rather than an action that isolates individuals and
links them together for the inevitable "reeling" that seems to always come from it, has
been reduced to  an action that has no expectations of anything past the moment.  In
order to make sure this happens, promiscuity is not only tolerated, but also encouraged
... going "out" with the same man over and over again could lead to attachments and do
the community harm ... and the community is much more important than the momentary
happiness of an individual.

Sex serves much the
same purpose as soma ... don't let an individual feel anything and you help assure the
stability of the whole.

"When the individual feels, the
community reels."