Composing a doctrinal statement (or any other essential documents) can be one of the most arduous (but crucial) projects undertaken by a church. In this series, I’m sharing my own doctrinal statement in an attempt to provide a helpful example of a detailed statement that is worded positively, but articulated precisely enough to exclude certain theological positions for the protection and unity of the church.
Marriage: We believe that the only biblical marriage is the formal union of a man and a woman in a lifelong, exclusive, comprehensive covenant. 
(Genesis 2:24; Malachi 2:14–16; Matthew 19:4–6; Mark 10:6–9; Romans 7:2–3; 1 Corinthians 7:10–11, 39; Ephesians 5:22–33)
Human Sexuality: We believe that any other sexual activity, identity, or expression outside of this definition of a biblical marriage, including those that are becoming more accepted in the culture and the courts, are contrary to God’s natural design and purpose for sexual activity, and thus are sinful. Any form of sexual perversion such as (but not limited to ) fornication, adultery, incest, homosexuality, bisexuality, bestiality, pedophilia, pornography, any attempt to change one’s sex or gender, or disagreement with one’s biological sex, are sinful perversions of God’s gift of sex, gender, and marriage. God has created us male and female, and he desires that we find joy and contentment in His design.
We believe that gender is God-given, not socially constructed or self-determined. Gender distinctions are rooted in creation, and manifested in biological, emotional, and constitutional differences . Being created as a man or woman is an essential  aspect of our identity, transcending social customs and cultural stereotypes.
(Genesis 2:18–25; Exodus 20:14; Leviticus 18:1–30; Matt 19:4–5; Mark 10:6–9; Romans 1:26–29; 1 Corinthians 5:1; 6:9–10; 1 Thessalonians 4:3–8; Hebrews 13:4; Jude 7)
1] Having a clear, biblical definition of marriage will not make your church popular, but it will mean standing on the authority of the Word of God and not compromising truth for approval. Having clear statements on marriage and sexuality also serve to protect the church in matters such as hiring staff and hosting weddings, and are the first line of defense against related legal issues. If you have a simple policy that anyone the church hires must agree with and conform to the church’s doctrinal statement, you avoid alot of agony in court. If you have a facilities use policy that the church building is not to be used for anything that goes against the church’s doctrine, then you protect yourself from lawsuits for refusing to host homosexual weddings and the like. At least right now, this is still an effective means of legal protection for the church. The day is coming very soon when churches will lose tax-exempt status over these issues. But for now at least, why not use the simple provisions our legal system has in place (left over from a time when the government thought that freedom of religion was something worth protecting, and that churches were a good to society) to protect your church from unneeded attack and hardship in these moments before the unavoidable persecution arrives? Here is a great resource on the matter.
2] It’s helpful, but not necessary, to have a list of some specific things you’re referring to, though there is no way to mention every variety of sexual sin individually, but we acknowledge that we live in a Romans 1 society in which people are inventing new ways to distort God’s design every day. The best way to cover it all is to say that any sexual activity outside of a biblical marriage is sinful.
3] That is, the makeup of maleness and femaleness is fundamentally different at the foundational, essential (see next note) level.
4] I’m using “essential” here in the technical sense of the word — that is, not to mean “really important,” but rather having to do with one’s essence, one’s ontology. In other words, we are not just created as humans, we are created as male or female humans.