For example, Christians stand on the word of God when we say that marriage is a life-long union of one man and one woman (e.g., Matthew 19:4-6), and that any sex outside of this union is a sin and a perversion of God's gifts of sex and marriage (e.g., Hebrews 13:4).
Critics say that if we cling to God's condemnation of homosexual behavior in Leviticus 18:22 (it's also in Romans 1:26-27 and 1 Corinthians 6:9-10), then we also are to follow God's condemnations of unclean foods and condemn ourselves when we eat bacon and ham (Leviticus 11:1-7), or that we condemn ourselves when we wear clothing which is made with a poly-cotton blend (Leviticus 19:19), and so on.
While such things can be refuted with several more paragraphs of script from me, I will refer to you Lutheran Satire which makes enough points to do the job. These 6 or so minutes will also be more entertaining than my typing. [Spoiler alert: We don't live under the Old Testament covenant anymore.]