Yes, the proclamation supports strong families, but not just one kind of strong family. Yes, it says "marriage between a man and a woman is ordained of God..." but it does not say that other unions are invalid.
For example we know that marriages between one man and multiple women are also viable on the other side of the veil. The proclamation does not invalidate polygamous marriages made prior to 1890 or the present-day serial polygamist who is sealed to one wife, and then upon her death is sealed to a second, resulting an eternal polygamist marriage.
The Proclamation on the Family speaks of many ideals. Yes, it certainly is ideal for a man and a woman to marry and rear children. No argument there. But it clearly and directly acknowledges that this ideal is not always attained because of a variety of reasons:
"Disability, death, or other circumstances may necessitate individual adaptation."
It doesn't say that other circumstances may cause problems but there's nothing that can be done. It says other circumstances may necessitate individual adaptation. That powerful, inclusive sentence is a loving acknowlegement that the ideals set forth in the proclamation are sometimes unavoidably unavailable to real people in real life.
In 1922 when my grandmother was widowed as a young mother, her parents did not allow her to date after the tragic loss of her young husband. They believed that dating would violate her Temple covenants with her eternal companion. Today she would not be so tightly constrained. Today she would be encouraged to marry a second worthy husband in the Temple, but not for time and all eternity. The doctrine hasn't changed, but the intrepretation of it has. It didn't even require a revelation, just a change in attitude and, perhaps, church policy. Does anyone how if there was an official change or not?
After the 1978 revelation opening the priesthood to worthy black members of the church, Bruce R. McConkie was questioned about his many strident statements against "the Negro." In a speech entitled All Are Alike unto God he said, "Forget everything that I have said, or what President Brigham Young or President George Q. Cannon or whomsoever has said in days past that is contrary to the present revelation. We spoke with a limited understanding and without the light and knowledge that now has come into the world. We get our truth and our light line upon line and precept upon precept. We have now had added a new flood of intelligence and light on this particular subject, and it erases all the darkness and all the views and all the thoughts of the past. They don’t matter any more."
Elder McConkie thus affirmed the Ninth Article of Faith: "We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God."
Fortunately we won't have to forget or set aside anything in the Proclamation on the Family if and when it is revealed that in addition to traditional marriage being ordained of God, "other circumstances" require individual and church-wide adaptation, and should be handled with as much love, accomodation and support as death or disability.
I believe that someday we will all sing with great fervor and thanksgiving Carol Lynn Pearson and Reid N. Nibley's prophetic Primary song I'll Walk with You:
If you don't walk as most people do,
Some people walk away from you,
But I won't! I won't!
If you don't talk as most people do
Some people talk and laugh at you,
But I won't! I won't!
I'll walk with you,
I'll talk with you.
That's how I'll show my love for you.
Jesus walked away from none.
He gave his love to ev'ryone.
So I will! I will!
Jesus blessed all he could see,
Then turned and said,
"Come, follow me."
And I will! I will! I will! I will!
I'll walk with you,
I'll talk with you.
That's how I'll show my love for you.