Question: "What does the Bible say about an open marriage? Does the Bible address polyamory/swinging?"
Answer: An open marriage is generally defined as a marriage in which one or both spouses are allowed by the other spouse to have sex with other people. The two primary types of open marriages are polyamory and swinging. Polyamory is when the extra-marital affairs purportedly involve emotional love. Swinging is when the extra-marital affairs only involve recreational/casual sex.
No, the Bible nowhere explicitly addresses polyamory, swinging, or the idea of an open marriage. The idea that one spouse would consent to the other spouse having sex with other people is absolutely foreign to the Bible. The Bible speaks of sex within marriage as pure (Hebrews 13:4). The Bible speaks of sex outside of marriage as immoral and adulterous (1 Corinthians 6:13, 18; 10:8; Galatians 5:19; Ephesians 5:3; Colossians 3:5; 1 Thessalonians 4:3).
The question is sometimes raised as to whether a polyamorous relationship should be considered adultery if the other spouse allows, approves, or even participates in it. The answer is an unequivocal yes! God is the one who defines what marriage is and what adultery is. God, in His Word, has declared sex outside of marriage to be adultery (Exodus 20:14). A spouse’s giving permission to sin does not overrule God’s Law. We do not have the authority to create exceptions to what God has declared to be sinful.
Aside from the biblical declarations that they are sin, polyamorous relationships cannot fulfill what the Bible says a marriage is to be. A married couple cannot be “one flesh” (Genesis 2:24) if multiple “fleshes” are involved. A married couple cannot fully love one another if that love is divided among other people. There cannot be true intimacy if what is supposed to be intimate is shared with others. Polyamory is not marriage. In no sense is a marriage supposed to be open to sexual activity outside of the marriage.
Polyamory is, in reality, “poly-lust-ory.” There is nothing loving about it. This perversion of marriage is confirmation that “every intention of the thoughts of our hearts is only evil continually,” and that, without God, “everyone does what is right in his own eyes” (see Genesis 6:5 and Judges 21:25).