Here in Australia we're in the throes of debate about same-gender marriage. One of the issues in the debate is: from the secular side: "what right do Christians have to dictate how other people live?"; and from the Christian side, the exact same question in reverse: "why bother trying to impose Christian values on non-Christians? They're unforgiven sinners anyway; just let them do what they want."
Here's some thoughts on why Christians care about other people's marriages. Not specific to the same-gender marriage debate, though of course it has implications for it.
1. Biblically speaking, God instituted marriage in Genesis ch 1 & 2. It's an aspect of being created male & female, for procreation - babies - and ruling the earth. Jesus validates heterosexual lifelong monogamous marriage in Matthew ch 19 (and parallels). There is a consistent Biblical metaphor of God and his people being compared to a groom and his bride, eg: Ezekiel 16; Hosea ch 1-3; Ephesians 5. So marriage is a “creation ordinance”, not a “gospel ordinance”. It’s good for everyone, not just Christians.
2. The Biblical picture of marriage is NOT about self-satisfaction, but about responsible service. God doesn't (metaphorically) "marry" his people because they make him feel good; but because he cares about them, takes pity on them, and wants to bless them. Similarly, human marriages are NOT about "baby, you make me feel good", but "for better or worse, richer or poorer, sickness and health" etc.
3. That means Biblical love is the opposite to modern secular love.
(a) Modern secular love is an expression of freedom: we MUST be free to "love" whoever or whatever we want, just because we feel like it.
(b) Biblical love is the opposite: it's about UNfreedom, commitment, binding ourselves to care for someone else whether we like it or not- ESPECIALLY when we don't feel like it.
4. Unsurprisingly, that means the Biblical view of what is "good" for us is exactly opposed to the secular view.
(a) The Bible thinks it's good for our sexual desires to be managed - controlled, directed - and marriage is one way to control our sexual desires. In marriage, we deliberately limit our freedom by subjugating our personal freedom to our words of promise, our commitment to our marriage partner. This is another reason why Christians affirm lifelong heterosexual monogamous marriage for anyone, Christian or not. It's a good external discipline, which channels our desires away from potentially unhealthy, destructive ways of expression, and towards healthy, constructive means of expression.
(b) The secular view of human "goodness" is unlimited freedom, ESPECIALLY sexual freedom. We must be free to do whatever we want, whatever we "feel" like. And anyone who tries to stop us is "oppressing" us. People rightly see marriage as limiting that freedom - so why get married? Just live with the person / people / animal / thing you like to have sex with. And when you stop liking to have sex with he / she / it, move on.
5. Unlimited freedom doesn't work. ESPECIALLY unlimited sexual freedom. Even atheist social commentators recognise that. See my review of Clive Hamilton's book "The Freedom Paradox".
6. This is why Christians stand for lifelong heterosexual monogamous marriage. It's actually good for individuals, families, society, and the world. We make a stand for lifelong heterosexual monogamous marriage because we love everyone, Christian or not.
7. This is also why no-one understands us and will call us hate-mongers. Sin is not rational - by it's nature, it's irrational and unhealthy. It is irrational and unhealthy to reject the creator God, who gave us all things - including our bodies, with their sexual possibilities - and loves us so much that he gave us principles and guidelines for the good use of those good bodies. So we shouldn't be surprised when people irrationally reject God's good purposes for ourselves, including our sexuality, and oppose those who try to make a stand for health and rationality.
8. And this is also why we have to patiently turn the other cheek and pray for those who persecute us. When we irrationally rejected God, he did not reject us, but sent his Son to die and rise for sinners. Similarly, when people irrationally reject and persecute us for caring for them, we don't reject them, but continue to love them anyway.