Not merely the love of one person, but the animal instinct, the simple undifferentiated desire: that was the force that would tear the Party to...

Basically it's saying that being human can take down the Party.
The party can't control the basic instinct that is ingrained in all of us - the need for
connection, love, pleasure etc. The thing that the Party fears the most is not being able to
control everything/everyone - and control is everything. So the Party tries to by making sex
something percieved as only a duty and otherwise a very "dirty" thing. It then stems all of the
unused energy (that would've otherwise been used for sex) into rallies and whatnot for Party
propoganda (a point raised by Julia somewhere in part 2 of the
book).

Another thing is that supports my beginning statement is when
Winston writes "If there is any hope, it lies in the proles" and then later he says (paraphrased)
"the proles are human. we are hardened, not human". And therefore they are the force that takes
down the Party. The Proles are people who are basically just normal people, and their lives are
hardly controlled by the Party in terms of thought police etc. So yeah they take down the
Party.

That's just a simple take on what that quote means (to me).
That animal instincts (AKA sex) is a lapse in the Party's control and therefore a way to bring
down the Party.