These people are violating the natural order established by God, and when someone does this it is considered a sin. Marriage was created by God, it wasn't created in a state or a room full of people in favor of marriage equality, He created it when He first created Adam and Eve. In the book of Genesis it says that “God created man in his image; in the divine image he created him; male and female he created them. God blessed them, saying, 'Be fruitful and multiply; fill the earth and subdue it" (Gen 1,28-29). Genesis also teaches us how God punished Sodom and Gomorrah for the sin of homosexuality and tells us how God rained fire on the two for the atrocities they had committed.