To answer this question "Why are Christians against gay marriages" one must look to an external source for truth. This truth comes from something that is not related to humanistic thought. The source is objective and it is the Bible.
Christians are against gay marriages because these marriages are against the nature that God has created. Christians and non-christians alike are made in the image of God (Genesis 1:26-27, 9:6).
Humankind surrendered their natural state (the image in which they were created) to worship themselves rather than God. This behavior is against the purposes and intentions of God. Romans 1:24-27 says, "Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. They exchanged the truth of God for a lie, and worshiped and served created things rather than the Creator - who is forever praised. Amen. Because of this, God gave them over to shameful lusts. Even their women exchanged natural relations for unnatural ones. In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion."
God's intention was for sexual pleasure between a man and woman (in a monogamous marital relationship), also for reproductive purposes. Gay marriages go against what God intended and this is another reason why Christians are against gay marriages. Genesis 2:22-24 lays out God's plan for marriage: "Then the LORD God made a woman from the rib he had taken out of the man, and he brought her to the man. The man said, 'This is now bone of my bones and flesh of my flesh; she shall be called "woman," for she was taken out of man.' For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh."
God did not make man for man in this context, God made the woman for the man. Christians are against gay marriages because it brings defilement into marriage (Hebrews 13:4).
Christians are against gay marriages because it is a lie, and people are deceived. God did not create anyone homosexual. It is a lifestyle that a person chooses.
If you are homosexual, we encourage you to look deep inside yourself. Are you joyful and happy? Look at your anatomy, what do you see? This is who you are created to be! There is help, just a prayer away. Acknowledge your situation, confess it, and turn to Jesus. He is there. Secure support from others who have been in your situation and have escaped the lifestyle