Sometimes, it is useful to try to express an argument in new ways. What is confusing in one expression may be clear in another.
So, let's take:
Here's an argument against desirism.
1. The majority of people desire to see the desire of a certain minority thwarted.(premise)
2.The desire to thwart the desires of the minority tends to fulfill more desires then it thwarts. (from 1-there are more desires that the desires of the minority be thwarted then there are that the desires of the minority not be thwarted.
3. The desire to thwart the desires of the minority is a good desire (from 2 and definition of good desire.
3. The right act is the one a person with good desires would perform.
4. A person with good desires would have the desire to thwart the desires of the minority. (From 2 and 3)
5. The right act is to thwart the desires of the minority. (From 3 and 4)
I suppose the committed desirist could accept the conclusion, and say that our moral intuitions are wrong in this case.
What does the term "people" mean in the first premise? Are these human beings, or a strange form of life discovered on Proxima Centauri?
If they are human beings, then the property of "the majority desire to see the desire of a certain minority thwarted" is a contingent fact. Even if it happens to be true at a given moment, we can ask what reasons exist to preserve this truth. What attitude should people adopt towards this desire?
The members of the minority have reason to condemn it.
Anybody who forms a friendship or other caring relationship with a member of this minority has reason to condemn it.
But, most important, among humans there is a great deal of potential for people harming others. Wars, terrorism, murder, rape, assault, abuse, theft, embezzlement, exploitation, fraud. To combat these, people generally have many and strong reasons to promote an overall aversion to thwarting the interests of others and to even offer aid in helping to fulfill those desires. These are reasons to condemn the attitudes expressed in Premise 1 and to condemn - perhaps even to punish - those who act on such a desire.
In other words, even if (1) is true, people have many and strong reasons to put the tools of condemnation and punishment to work to make it false. There are no reasons being offered to keep it true.
Against this, one might assert, "But this is a premise. You are to just assume that it is true."
Well, if I am being instructed to accept this premise then I am being instructed to not only accept that the majority have this desire, I am also being instructed to assume that the attitude in this case is legitimate and cannot be criticized or condemned. The argument smuggles in a moral premise that it is not only a fact that the majority desires to thwart the desires of the minority, but it is a morally neutral fact.
This unspoken moral claim, smuggled into the argument, assumes that Desirism is false. Since the argument is intended as an argument against Desirism, in this form, it is begging the question.
So, if the attitude expressed in (1) cannot be morally condemned, then it begs the question against Desirism. If it can be criticized, Desirism points to many and strong reasons to condemn it. Those reasons do not bring in an intrinsic "ought not to be doneness". They are built on the actual reasons of those who are a member of the minority, those who care for members of the minority, and reasons to promote kindness and a general intolerance of cruelty.
The next maneuver that I can imagine is to say, "No. Imagine not only that the majority have this desire to thwart the desires of a minority, but that they are always kind and helpful to each other. Consequently, there is no reason to promote a general desire to help others or to condemn unkindness and cruelty that would otherwise provide a reason to condemn the cruelty towards this minority."
If we go this route, then we are abandoning the assumption that "people" means "humans" and entering a realm where "people" means "an alien race found living on proxima centauri who are significantly different from humans." Under this assumption, I would argue that we need a science fiction author to flesh out the details of this alien race before we can draw any conclusions about what they should or should not do.
However, I can say that our reasons for promoting an aversion to cruelty and a disposition to help others would give us reason to side with the minority on this alien planet and at least pull us in the direction of doing something to better their welfare and end their victimization. We retain our reasons for condemning this cruelty towards the minority even if the majority on this planet lack any reasons to do so. The fact that we can imagine a possible world in which people are being victimized does not imply that we have a reason to adopt a neutral attitude towards that world. We may still have reasons to adopt an attitude of condemnation and to promote that attitude among other humans.
So, in summary
If "person" means "human" then either I can condemn the attitude in Premise 1 or I cannot. If I cannot, then you are smuggling a moral claim into the argument that contradicts (begs the question against) desirism. If I can condemn the attitude in Premise 1, I will - because there are many and strong reasons among humans to do so. If we are also going to introduce a premise that says that there are no reasons to condemn the attitude in Premise 1, then we must deny the assumption that "person" means "human" and we are talking about attitudes among an alien species. We do not have enough information to make judgments about what they should do - but it is still the case that we have reason to promote an aversion to their cruelty among fellow humans.
In other words, the right act among humans is to condemn the people in premise (1) who have this desire to thwart the desires of the minority.