Is the Party from 1984 right about making assumptions of gender?
Your question is worded in a difficult manner as you use the term "right" about unspecified assumptions in the novel, so I must make an attempt to answer as best I can.
The "party" or more specifically, the "inner party" of the novel are the controllers of the mechanism of society. They have built an effective dystopia in the years they have managed history and created a society of people controlled by them. One of the common threads in dystopian literature and other forms of art is the distortion of sexuality. In Brave New World, for example, human sexuality is used as a means of pacifying the people and as entertainment and little else, much like the drug used to pacify people. In 1984 the means of control is vastly different, yet still a distortion of what we now consider to be normal sexuality. The inner party must assume that the bonding which occurs between persons through sexual contact is dangerous as it forms a tie between individuals; hence, any tie between individuals is dangerous to a system which relies on totalitarian unity of people and rampant suspicion between them. The "Anti-Sex" league is an example of the systematic programming of children and young adults to reject sex as anything other than utilitarian for the purposes of expanding population for use by the state. Ayn Rand's novel Anthem has a similar view.
Thus, I conclude that Orwell's tactic in the novel is to point out the unlimited power of the state to pervert even normal biological functions to the use of the state. Since he was seeing the Soviet expansion of Marxist / Leninist communism and the pogroms of Stalin as the potential future of England, he would have seen that such an evolving society would necessarily have to deal with all the ties that bind people together other than the supreme bond of the state. Hence, dystopian societies throughout literature seem to manipulate gender for the purpose of control. Is this "right?" as you ask? Well, from the state's point of view in the novel, it is both right and essential, but from the viewpoint of modern readers it is a horrific perversion of the human norm, and so must be considered "wrong."