Love and marriage are extremely important to the Party, in a negative sense. In other words, they see love and the sort of marriage based on love as inherently subversive forces and attempt to regulate them and suppress them. They want the full force of people's emotions directed in ways that directly benefit the Party, first as love for Big Brother and second as rage and hatred directed against opponents of the Party. As O'Brien says:
There is no loyalty but loyalty to the Party. There is no love except love of Big Brother. All competing pleasures we will destroy.
Thus the Party destroys the family, trying to ensure that parents and children have no affection for each other and forbids sex for any purpose but procreation. O'Brien even lets slip that they are working on a way to prevent people from experiencing orgasms, as that sort of intense physical pleasure might be something people would strive for in a way that would interfere with single-minded loyalty to the Party. Winston's affair with Julia is thus viewed as treasonous and subversive.