In the Hebrews passage I noticed that 13:4 tells us that "marriage should be held in honor among all..." And so from thinking about these two commands in Scripture I come to some conclusions, and maybe they raise other questions.
The marriage of (any and all) heterosexual partners is something God establishes. The passage in Mark certainly makes a positive statement that marriage is between a man and a woman, as there were no other options in the garden, and this was obviously by God's design. The fact that the marriage ceremony has varied over the millenia, being something between families, within a village or community, being celebrated with feast, gifts, and ceremony, given "official" status by the church, and then by the state doesn't take away from a theological understanding of every marriage. It is God ordained, and that ordination is active in the institution generally and in each marital relationship severally. God does the joining, and it is consummated in the two becoming one through the sexual act of intercourse, and they are no longer two but one. This act of becoming one cannot happen between homosexuals and therefore there is no marriage there, certainly not one ordained or joined by God, no matter what any earthly authority might say.
Marriage is not simply the sex act, it is a covenant primarily between the two individuals involved, consummated and celebrated in the physical union, and is to be steadfastly protected and honored by everyone. Marriage is not something simply to be honored by immediate family, or local community, but by anyone and everyone in the whole world when they meet and interact with a married couple. The Egyptian Pharaoh doesn't have a right to take Abraham's wife, even if he thinks she is Abraham's sister, and even if Sarah is not from Egypt and was not married according to Egyptian rules, whatever they may have been.
It is not wrong for the church and the state to license, celebrate,and regulate marriage, as long as it doesn't make it more or less than God has declared. Children, property, inheritance, and health all are involved in the proper recognition and regulation of marriage relationships. It is proper for the community to celebrate weddings, not simply to provide historic photographs, but for those of us who attend the event to witness this covenant making and to begin to honor this new marriage relationship and support it. We support each marriage not simply as a love for an individual couple, but as an act of support for all couples.
Each marriage has its own struggles, trials, and temptations. We make mention of that reality in the vows we use in the ceremony. Some of this struggle comes from the common weakness of the individuals involved, some struggles will arise from the circumstances couples will variously face as they go through life, and some struggles will come from the interference of others. If we are to honor marriage we must protect it, for all of us, and for each of us.
So the question arises, what about those marriages that should not have happened? Suppose a Christian marries a non-Christian; once married is that marriage joined by God? We know that when Jesus comes into a person's life, and that person is already married to someone who doesn't want Jesus, the person not loving Jesus is free to leave the Christian. The Christian however is not free to leave the non-christian. If the believer purposefully marries a non-christian we don't think they then have an option to leave when they realize what they have done and want out.
I am not asking here about those who feel they made a mistake due to a growing awareness of incompatibility. That could be the "get out of jail card" for a lot of couples at various moments of their married life. Marriage is considered by most Christians to be so radical a commitment that even those marriages that "should not have been" have to be protected, and are considered under the command of Jesus not to be interfered with. I am not speaking here of incestuous or child bondage brides or any that are against the law, in and under the proper regulation of the state.
My comments here are not addressed to the boundaries of what might be called a 'Biblical divorce." That is another discussion.