The reformers rejected the sacrament of marriage and turned marriage over to the civil authorities. (I understand that reformation churches even stopped conducting weddings for a few centuries.) Today, many Christians object to how the civil authorities are redefining marriage. But if marriage is under the purview of the civil authorities, how can this be a valid objection? Should churches take marriage back from the civil authorities?
If marriage is not a sacrament, then what is it?
At the start of the Church of England wedding service, the minister refers to marriage as having been "instituted by God in the time of man's innocency". Marriage is universal, and a gift of God to all mankind. Even those who believe marriage can be a sacrament, generally do not believe it is always so.
Luther saw marriage as a civil contract blessed by God. He recommended that ministers pose questions to the couple at the Church door and that they there make their promises. They were then led into the church whereupon the marriage received blessing.
A civil contract is not the same as a private understanding. A contract is enforceable at law, by either of the parties but also sometimes by society at large.
Some Christians object to redefinition of marriage where the redefinition involves a loosening of the ties that bind. An agreement terminable at will by either of the parties is not the kind of civil contract Luther had in mind. The very concept of marriage entailing a civil contract implies that such a contract is legally permitted. In many jurisdictions it is impossible to make a legally binding permanent marriage contract, because the state will not uphold it. Some Christians believe divorce should not be an option, or should be available only in very restricted circumstances. This is not necessarily to deny the right of non-Christians to enter into temporary agreements of various kinds if they wish; it is about claiming the right to make a particular type of contract, whose terms have historically been meant by "marriage".
Another issue is that many Christians believe that the permanent union of a man and a woman is the foundation of human society, and that we need a word for this special relationship. If the word "marriage" comes to mean something wider, then we no longer have a word referring specifically to this special and particular relationship of husband and wife. This then leads to a reduced understanding of the importance of traditional marriage.
Another way marriage can be re-defined is to include couples of the same sex. One objection to this is that again it means we no longer have a word for traditional marriage. Another is that the sexual aspects which may be involved are regarded, by some Christians, as sinful. That is not to say that covenants such as that between David and Jonathan are objectionable.
A concern in some countries is that if marriage between persons of the same sex is allowed, and if discrimination on grounds of sexuality is not allowed, then churches and ministers could be forced to conduct same sex weddings, or sued for not doing. Established churches may be especially vulnerable since refusal of marriage there might in the future be construed as state discrimination, on which laws may be especially strong.
There are Christian and non-Christian views for and against, but the question asks only for Christian views against redefinition.
The statement in the body of the question that Reformation churches generally did not conduct weddings is not correct. Independent denominations usually did not conduct weddings, but they were customarily conducted in the official churches.
To summarise: it is precisely because of the Protestant understanding of marriage as a civil contract, that many Protestants believe that a civil contract, meeting the requirements of Christian marriage, should exist and be available.
I'm not too familiar with sacraments, but in doing a quick reading of what a sacrament is, I came upon the following:
An outward sign of an inward grace that has been instituted by Jesus Christ. Sacraments signify God's grace in a way that is outwardly observable to the participant.
Using this definition I would agree that it is, as marriage was instituted by God Himself in the Garden of Eden, and then Jesus says:
For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh’; so then they are no longer two, but one flesh. Therefore what God has joined together, let not man separate.”
Jesus made it pretty evident that God joins together. And this joining has to do with the physical joining as well, sex. However, we do live in a world of sin where people "marry" one another as they please, without regard for God or what He meant this joining of marriage to be. Jesus prophesied that it would be like that in the final days before His return:
36 “But of that day and hour no one knows, not even the angels of [f]heaven, but My Father only. 37 But as the days of Noah were, so also will the coming of the Son of Man be. 38 For as in the days before the flood, they were eating and drinking, marrying and giving in marriage, until the day that Noah entered the ark, 39 and did not know until the flood came and took them all away, so also will the coming of the Son of Man be.
Civil marriage is NOT the Biblical marriage that Jesus was speaking about. It is simply a paper that allows certain civil rights as a civil union.
My opinion is that the separation of Church and State is a wise thing. I do not think Christians should be imposing our beliefs upon unbelievers through the force of law. We do not live in a Theocracy. I may not agree with what is going on, but it is prophesied that it would happen. God's Word and truth must be presented, but in love. Love is not forced or coerced. Using the law to take away someone's civil rights is simply because they believe differently is not correct.