In 1984, according to the Party what is the most harmful part of the sex act?

The Party is not opposed
to sexual activity because they consider sex somehow wrong or sinful. Instead, the opposition to
sex has to do with beliefs about how sexuality affects people's psychological states. It should
be noted that some of 's ideas which are reflected in the novel are based on classical Freudian
psychology and are not necessarily what 21st century scientists might believe. 

The first issue for the Party is that romantic or sexual relationships inspire love or
loyalty to something other than the Party and that this may lead to subversive thinking as the
lover starts to realize that there is something more important than the Party.

Next, the Party believes that the sort of energy required for fanatical devotion to the
Party flows in part from repressed sexuality (a version of classical Freudian theory). Thus the
Party attempts to channel repressed sexual energies into labor and hatred of "enemies"
of the Party, creating an energetic and fanatical cadre of party loyalists. 

astutely points this out in the following passage:

It was
not merely that the sex instinct created a world of its own which was outside the Party's
control and which therefore had to be destroyed if possible. What was more important was that
sexual privation induced hysteria, which was desirable because it could be transformed into
war-fever and leader-worship