Since Christ is the only other person from the Melchizedek priesthood, who is performing these ceremonies claiming to be from the priesthood of Melchizedek? And seeing as we don't have that much information about the Melchizedek priesthood, how do we know that this priesthood seals marriages?
We know the apostles were not preaching about eternal marriages
Romans 7:1-4
1 Or do you not know, brethren (for I speak to those who know the law), that the law has dominion over a man as long as he lives? 2 For the woman who has a husband is bound by the law to her husband as long as he lives. But if the husband dies, she is released from the law of her husband. 3 So then if, while her husband lives, she marries another man, she will be called an adulteress; but if her husband dies, she is free from that law, so that she is no adulteress, though she has married another man. 4 Therefore, my brethren, you also have become dead to the law through the body of Christ, that you may be married to anotherto Him who was raised from the dead, that we should bear fruit to God.
and
1 Corinthians 7:29
39 A wife is bound by law as long as her husband lives; but if her husband dies, she is at liberty to be married to whom she wishes, only in the Lord.
If Christ restored this to Peter, why did they not preach on it?
The only priesthood that the Sadducees would have known would have been the Priesthood of Aaron, or the Levitical priesthood. The Aaronic priesthood does not have the authority to seal marriages forever to begin with.
The Sadducees, who did not even believe in the resurrection, were trying to trip Jesus up by asking what they felt was a contradictory question. In it, they totally expected him to mess up trying to answer the question that they felt had no real answer under the law of Moses (see verse 24).
Christ points out a couple problems with their question: That they "do not know the scriptures" and the "power of God". They did not know the priesthood that Jesus Christ holds, and had no idea what they were talking about.
Secondly, Christ points out that in the resurrection, it is too late to figure out who is going to be with who. All that needs to be settled now. "For in the resurrection they neither marry, nor are given in marriage..."
So, since the question was premised on the law of Moses, and the Aaronic priesthood, she would be married to none of them in the resurrection, and they would all be angels. Further, it is too late, when the resurrection occurs, to marry or be given in marriage.
Christ answered their questions. He said "in the ressurrection they neither marry nor give into marriage". He didn't tell them that another priesthood is going to come and change that, His apostles didn't even teach such a thing. He made it sound like this is a future thing that will happen when we resurrect and go home. And the scripture said they will be "like the angels" when it comes to marriage.
And seeing as the church is the body of Christ and not a temple made by hands, where does the idea come from that we need to be sealed in a temple?
Under the law of Christ, We must be sealed to our spouses by the priesthood of Melchizedek, while still in mortality, in the temple, if we want our marriages to endure death and be in force when we are resurrected.
I don't understand what you mean by temple. Are you talking about a building or the body? The scripture tells us that the church is members, body of Christ, and it is not a temple in which we dwell.
That's our take on it anyway.
The main reason we believe in this principle, however, is because we sincerely believe that God has called prophets and apostles again in our day, and has restored this knowledge to us through them. Our understanding jives with the scripture that says, neither is the man without the woman, neither the woman without the man, in the Lord. (1 Cor. 11:11)
Sincerely,
Vanhin
as for the verse in 1 Corinthians 11:11, I think that is dealing with the authority and head covering, I'm not so sure it deals with husband and wife. See, put it in its context:
1 Corinthians 11:10-15
10 For this reason the woman ought to have a symbol of authority on her head, because of the angels. 11 Nevertheless, neither is man independent of woman, nor woman independent of man, in the Lord. 12 For as woman came from man, even so man also comes through woman; but all things are from God.
13 Judge among yourselves. Is it proper for a woman to pray to God with her head uncovered? 14 Does not even nature itself teach you that if a man has long hair, it is a dishonor to him? 15 But if a woman has long hair, it is a glory to her; for her hair is given to her for a covering.