God created marriage.
When GOD created Adam, there was no suitable companion among all of GOD's other creatures. So GOD made woman from one of Adam's ribs. Adam called Eve woman because she was taken from man.
Immediately following this, we read:
24 For this reason a man shall leave his father and mother and be joined to his wife; and the two shall become one flesh.
(Genesis 2:24 OSB)
And so we have the first marriage in human history.
Marriage was created by GOD.
From the beginning, woman was created as a helper to man (see Genesis 2:18).
From the beginning, GOD made the marriage relationship between one man and one woman.
And from the beginning, GOD created the sexual relationship as a gift to the man and his wife. In sexual intercourse, a man and his wife become one flesh.
From the beginning, there was no formal ceremony or governmental documentation. What we had was a commitment between a man and a woman to join together as a family, to love each other and to share the gift of sex with only one another. This was pure marriage from GOD.
Over time, many cultural traditions formed around marriage. These are fine (insofar as they are consistent with GOD's will). Jesus participated in cultural wedding traditions during the first century. But these traditions aren't what make marriage valid in GOD's eyes. And it is GOD's view that matters.
Now, we have previously established that GOD designed proper order for all of creation—how everything was to function together. GOD has a will for how creation should function. Everything that aligns with GOD's will is holy; everything that is inconsistent with GOD's designed order is sin.
It doesn't matter what you and I think about GOD's designed order. We can love it; we can hate it. We can see things we feel should be different. You are free to take your complaint to GOD. You are free to rebel against GOD's designed order and do things your way if you choose. But nothing you do will change GOD's design. And rebelling will only end badly for you because GOD knows what is best (that's why He designed things to work the way they do ... because it's best).
Like many concepts, in recent years, societies around the world have sought to redifine marriage.
Many nations have passed laws allowing two men or two women to be seen as married—AKA homesexual marriage or gay marriage. Such redefinition is foolishness.
Let me be clear: The Bible unilaterally condemns all forms of homosexuality. However, when it comes to the redefinition of marriage, condemnation of homosexuality isn't the point.
While people can redefine the word "marriage" all they want, they cannot change the institution of marriage because the concept is GOD-given and GOD-defined. Marriage is GOD's foundation upon which biological procreation occurs. (Fundamentally, there's only one way to accomplish procreation.) Couples form families and many of those families produce children who continue the cycle.
Redefining marriage to include homosexual relationships is like renaming a duck to be called a bear. You can substitute "bear" for "duck" as much as you like, but the anatomy of the duck will still be a duck. It won't change. The only thing that changes is the terminology.
The concepts of woman being created for man and marriage excluding homosexual relationships run counter to many cultures today. This is nothing new. We live in a sin-cursed world where the majority—the popular thing to do—is always going to be inconsistent with GOD's design. As a species, humans want to do things our own way. We dislike being told how we should live, even when it's what's best for us.
The truth is never going to win a popularity contest, not in this life, not here. But just like that renaming that duck-bear, following after falsehoods does nothing to change the truth.
Marriage is GOD's gift. He defined it. He blessed it. There is nothing we can do to change what GOD has done.