It seems to me that it is morally good to treat other people not as objects but as subjects. Someone, I take it, has a morally distorted picture of reality if he or she treats other people as mere objects instead of subjects.
Now, Sartre argues that, as a matter of fact, we all try to reduce other people to objects and that everyone is busy precisely with avoiding to be reduced to an object by others. Nobody wants to be considered a mere object in the eyes of the other and everyone strives to be itself a subject by seeking to reduce others to mere objects. So, according to Sartre, the inclination to reduce other people to objects is inherent to our human nature. This might indeed be the case. But that does not make such behavior morally acceptable. I take it that it is morally wrong to try to reduce other people to objects, even if doing so belongs to our human nature.
The thesis that it is just rational to treat others as subjects and not objects is reminiscent to Kantian ethics. The ethical system of Kant culminates in his famous categorical imperative: "Act in such a way that you treat humanity, whether in your own person or in the person of any other, always at the same time as an end and never merely as a means". So, Kant's imperative requires that one should not treat a person merely as a means, i.e. one should treat others as ends or one should treat others as means that are also ends. Kant argues for this imperative by claiming that it is implied by our reason itself, that is, it is our own human rationality that learns us that this imperative is perfectly rational and therefore the proper way to act. Note that Kant's imperative doesn't tell us what to do, it tells us how to do it, i.e. it addresses our behavior.
There is another ethical position according to which one should treat others as subjects instead of objects. The position I have in mind is Christian ethics. It goes beyond a Kantian appeal to the principle that it is rational to bring ones actions in accordance with a cognitively correct picture of the world. Christian ethics is grounded in love instead of rationality. Christians would totally agree with Kant that treating others as subjects instead of objects is a way of behavior that is fully in accordance with a rationally undistorted picture of reality. However, according to Christian ethics, one should treat others as subjects not just because doing so is implied by our reason, but primarily because one loves other people. Surely, this love is not the same kind of love one feels for its spouse, children, other family, or friends. Nor has it anything to do with eros. The love meant here is known as agape or caritas. It is reflected for example in these words of Jesus of Nazareth: "A new command I give you: Love one another. As I have loved you, so you must love one another". So, Christianity addresses our way of being. It's about being involved in others and caring about them. We treat others as subjects because we are truly interested in them, not because it is rational to do so.
For me the main question would be this: should we embrace a kind of rational Kantian ethics according to which we treat others as subjects instead of objects because our reason implies that this is the most rational conduct? Or, should we aim at the more profound far-reaching goal of agape or caritas? The first goal should be attainable for most people. The second goal might be much more difficult to arrive at for many (perhaps most?) people.