Day after day, hundreds of people around the world join together in the institution of marriage, some legally covenanting together, and unfortunately, others not so, but illegally through cohabitation. In either case, this just shows the longing heart within human beings on the need to be together with someone else for the purpose of companionship. Why is this institution very important? In order to give a proper answer, we shall look to what the Bible says about what marriage is so that we can have a proper theology of marriage, and then also counter the claims made by adherents of same-sex marriage.
The first instance of marriage comes in the early chapters of Genesis. In chapter 1, we’re given a telescopic and general account of creation, and in verse 27 we can see that they were created male and female. However, come in chapter 2, we are given an expanded microscopic and specific account of creation whereby in verse 7 we can see that man (Adam) was formed first from the ground, however, unlike other creatures which were male and female, man was only by himself as we can see in verse 18, and then God created a woman (Eve) to be a helpmeet for man, and in that context we can see the first wedding ever that was sanctioned by God Himself for the good of human beings, so that they may glorify Him through it.
From this chapter, we can learn a few lessons about marriage: first, it is for companionship purposes, all creatures were great, but none could provide the longing of companionship that man needed, and in order to fulfill this need, God created and brought Eve, the complete and suitable companion to Adam. Secondly, in verse 24 we can see that marriage is for the intent of committed union and devotion between the man and the woman who become one flesh, and form a new family unit separate from the families they’ve come from. Thirdly, we can see that marriage was also for the purpose of procreation as seen in Genesis 1:28 when God blessed them to be fruitful and multiply, so that the Earth would be filled with the image of God, thus His (God’s) glory covering it. This need will continue as long only in this present Earth (even during the Millennial Kingdom) but after it, during the eternal state, there won’t be procreation, as there will also not be marriages as Jesus told His disciples in Matthew 22:30. Another purpose for marriage is to point to the union between Christ Jesus and His bride, the church, Ephesians 5: 31-32. Lastly, marriage also is intended for the purpose of preventing immorality in the society, 1st Corinthians 7:2. All these purposes can be summarized that indeed it is a blessing as it is a grace of life as Peter tells us in 1st Peter 3:7.
On the other hand, over the past few years, there has been an explosion for the affirmation of same-sex marriage even by ‘professed believers’. Looking at the Bible, the first instance of homosexuality appears in chapter 19 of Genesis, which led to judgment from God against the Canaanite cities of Sodom and Gomorrah. Some people who affirm same-sex marriage say that the reason for the judgment was due to inhospitality by the people of Sodom, however, standing with authorial intent, this is a bogus claim since proper reading of the text clearly shows that the men of the city didn’t intend to know Lot’s angelic guests in a common social way, but in a sexual way as evidenced seen verses 7 & 8 of Genesis 19, and that is why Lot offered his two daughters. Similar incident related to this can be seen in Judges 19 of that narrative of the Levite and his concubine at Gibeah. Also, as we examine the texts of the Scriptures whereby homosexuality is mentioned, it is always on the negative that God totally rejects, Leviticus 18:22, 20:13, and Romans 1:26-27. The Bible forbids ‘homosexual marriage’ as it against nature in that God created male and female, two genders, and not one, and totally stands against all the purposes of marriage which we saw earlier, i.e. for companionship, procreation, illustrating relationship between Christ and His Church, etc. and as MacArthur & Mayhue say, “homosexuality is not another option for two consenting adults, it is an aberration of God’s design”. 
1. MacArthur, John and Mayhue, Richard, Biblical Doctrine: A Systematic Summary of Bible Truth, Wheaton, Illinois: Crossway, 2017.