"The sexual act, successfully performed, was rebellion. Desire was thoughtcrime." Explain.Explain, with reference to the Party's beliefs &...

In this society, a major goal of the Party is to make sure
that people have no loyalty to anything but the Party.  To accomplish this, one thing it
does is to try to destroy personal feelings  and emotions other than the ones the Party
produces (Two Minutes Hate, for example).

So, if the Party
is trying to get you not to have feelings for other people, then sexual desire and true
sexual intimacy are things that go against what the Party wants (and that makes them
crimes).

Winston says that this is why his life married to
Katharine was not satisfying (emotionally and sexually).  She was too conditioned by the
Party to want to sleep with him.