Question: Isn't the Mormon opposition to same-sex marriage hypocritical, considering that they used to ban black from holding the priesthood until 1978?

The Law of Chastity is doctrine with scriptural precedent, whereas the priesthood ban was a practice that was always said to be temporary

President McKay taught: <blockqutoe> There is not now, and there never has been a doctrine in this church that the negroes are under a divine curse. There is no doctrine in the church of any kind pertaining to the negro. We believe that we have a scriptural precedent for withholding the priesthood from the negro. It is a practice, not a doctrine, and the practice someday will be changed. And that's all there is to it. (Sterling M. McMurrin affidavit, March 6, 1979. See David O. McKay and the Rise of Modern Mormonism by Greg Prince and William Robert Wright. Quoted by Genesis Group)[1] </blockquote>

The priesthood ban was not based on a choice

Just because a black man was denied the priesthood before 1978, does not mean he did anything wrong. It was a practice that was applied to all black men and had nothing to do with the choices of the individual person. Being black was not a choice that he made. Following the law of chastity is a choice. Everyone can follow the law of chastity, regardless of sexual orientation. If someone chooses to have sexual relationships outside of a heterosexual marriage, that is a worthiness issue and is a choice that they are making.

It was prophesied that the priesthood ban would be reversed

It was prophesied that the priesthood ban would be reversed, whereas we are told the law of chastity would always be in place.

For example, in reference to black people, Brigham Young taught:

"The time will come when they will have the privilege of all we have the privilege of and more." (Brigham Young, Speech given in Joint Session of the Utah Legislature, February 5, 1952, in Fred Collier, The Teachings of President Brigham Young. Salt Lake City, Collier's Publishing, 1987, 43) Whereas President Hinckley taught:

Prophets of God have repeatedly taught through the ages that practices of homosexual relations, fornication, and adultery are grievous sins.[2]

The priesthood ban needed to be reversed so all of God's children could have the blessings of the highest degree of the Celestial Kingdom, whereas the Law of Chastity, as it stands, already allows all people these blessings.

Scriptural precedence

Jesus Christ taught that marriage is between a man and a woman, whereas He did not teach blacks would not receive the priesthood. (See Christ's teachings on homosexuality)
The Law of Chastity has scriptural precedence, whereas the priesthood ban did not.

Notes

Question: Isn't the Mormon opposition to same-sex marriage hypocritical, considering that they used to ban black from holding the priesthood until 1978?