The question is centuries old. Moral judgment could be broken into 2 schools of thinking: deontology, and consequentialism. A consequentialist would say that it didn't matter how the person did it, but now you're in love and very happy, so the act was not immoral. The ends justified the means. A deontologist would say that this was a horrible thing, because the means robbed you of your choice in the matter. Even if rejection ended in the person feeling horrible and worthless, he or she must let you make your choices. It makes no difference whether you are happy in the end, it was the method in which it was carried out that violated ethical action. The means are how the morality is measured.
The way most people lean tends to depend upon the situation, but I like this illustration of the importance of agency. Most people respond to this scenario by saying that it doesn't matter how much in love you ended up, the immoral thing was robbing you of the choice. Most healthy people, when they love another, at least want that person to have his or her agency.
It is for this reason that I find so many of Joseph Smith's proposals to be troubling. Consider the following from Helen Mar Kimball and decide if this young woman's agency is being manipulated:
Without any preliminaries, my father asked me if I would believe him if he told me that it was right for married men to take other wives. The first impulse was anger... My sensibilities were painfully touched. I felt such a sense of personal injury and displeasure; for to mention such a thing to me I thought altogether unworthy of my father, and as quick he spoke , I replied to him, short and emphatically, NO I WOULDN'T! This is the first time that I ever openly manifested anger towards him. Then he (my father) commenced talking seriously and reasoned and explained the principle (of polygamy) and why it was again established upon the earth, etc. This first interview had a similar effect to a sudden shock of a small earthquake. When he found (after the first outburst of displeasure for supposed injury) and I received it meekly, he took the first opportunity to introduce Sarah Ann to me as Joseph's Wife. This astonished me beyond measure. Having a great desire to be connected with the Prophet, Joseph, he (my father) offered me to him; this I afterward learned from the Prophet's own mouth. My father had but one Ewe Lamb, but willingly laid her upon the altar: how cruel this seemed to my mother whose heartstrings were already stretched until they were ready to snap asunder, for she had already taken Sarah Noon to wife and she thought she had made sufficient sacrifice but the Lord required more. (source)She later expressed the following: "I would never have been sealed to Joseph had I known it was anything more than ceremony. I was young, and they deceived me, by saying the salvation of our whole family depended on it." (quoted in Van Wagoner, 1992, p. 82)
And so how does this reflect upon Smith's respect for this young woman's agency? Is he justified because her entire family was now guaranteed eternal salvation: the benefits outweighing any negative feelings she had? Or is the method in which he convinced her to marry him a very real violation of Christlike morality and respect for agency?
This is also compounded by the fact that she was 14 years old (source)! If agency is what Christ died for, how could it be righteous and godly to coerce this young woman into marriage with a 37-year-old man? Is that how a perfect, merciful, loving god acts?
Post script: If Satan's goal is to have all of us be miserable (2 Nephi 2:27), it sure seems like Joseph Smith did a lot of his footwork in this instance.
ReferenceVan Wagoner, R. S. (1992). Mormon polygamy: A history. Salt Lake City, UT: Signature Books.