Heres my initial reaction:
Of course Elder Packer, and Apostle of the Lord Jesus Christ said homosexuality is a sin. Jesus, as Jehovah, said the same thing through his Old Testament prophets. He said the same thing through His New Testament prophets. He's said the same thing through his modern-day prophets. In fact, every prophetic utterance on this subject in Mormon scripture (which includes utterances from the beginning of time to the present) states homosexuality is a sin. This isn't new stuff. It's old stuff. Old as the hills.
It's equally normal that Elder Packer would say people with same gender attraction would be able to change. Since Mormon scripture states innumerable times the atonement of Christ can change you, your heart, and your actions, then it is without doubt you can change. Thus, Elder Packer's assertion is absolutely consonant with 1 Nephi 3:7:
And it came to pass that I, Nephi, said unto my father: I will go and do the things the Lord has commanded, for I know the Lord giveth no commandments unto the children of men, save he shall prepare a way for them that they may accomplish the thing which he commandeth them.
If we could not change and comply with God's standard for sexual morality, then the atonement is limited and 1 Nephi 3:7 is wrong. However, we know the atonement is not limited, and we know scripture is not wrong. Change by means of the atonement of Christ is possible for all sins, all temptations, all people. Gayness is not a special sin outside the reach of the atonement. It is but one sin among diverse sins, all of which are covered by Christ and His overarching love.
But, to be honest, these most disturbing reactions to Elder Packer's talk have come from active members of the Church. Many, in discussions with those critical of Elder Packer, have criticized him for his tone. They argue Elder Packer's tone was inappropriately harsh. He should have been more conciliatory; his opinions more muted, they argue. Really? His tone? This is an argument about style, not substance. These members are openly criticizing an Apostle for his tone when everything he said is absolutely consistent with every word of revealed scripture. We should spend less time whining about Elder Packer's tone and more time ruminating on the eternal truths Christ chose to reveal through him. Tone. You must be kidding me.
Ultimately, we would be well to remember how Joseph Smith acted when a woman came to him and told him the latest gossip about him. He said he listened to the woman and the terrible things others were saying about him, and instead of dismissing them as lies, he examined the gossip for any kernel of truth, and then took those kernels and used them to correct his actions and improve himself. Joseph cared not about tone; he cared about what was right. He cared about changing himself to become more like Christ. May we all do the same.