I get variations of this question fairly frequently. Are there other posts on my blog about a similar issue? Very likely. The short answer to the question is "no." Not all Christians are opposed to same-sex marriage. My slightly longer, more nuanced, answer is "no, but they should be." Let's unpack that a bit.
In the biblical account of creation, in Genesis 2:18-24, we can discern a complementary role of male and female. This is illustrated certainly in the world of animals, as well as in many plants. In the animal kingdom, there are only a few species for which both male and female are not needed to reproduce and continue the species.
In Matthew 19:3-6, Jesus is recorded as not only affirming the assumption that marriage, central to family formation, consists of male and female relationships, but also that the source of this understanding is in Genesis chapter two and that marriage is something intended to endure throughout life.
We are given a compelling rationale for this in the letter of the apostle Paul to the church at Ephesus, also included in the New Testament writings. In Ephesians 5:31-32 the apostle quotes Genesis 2:24 and says the real relationship he is describing is that of Christ and the Church.
Now here's where my slightly more nuanced answer from the first paragraph gets some traction. If marriage is to be a picture of Christ and the Church, it is not to be of two (or more) beings who are identical in all ways. As males and females have a fundamental equality but normally perform different roles, Christ has made the Church fundamentally his equal, through what is described as adoption. Yet Christ is the head of the Church, which is His body. There are different roles. Since marriage depicts this relationship, it is reasonable to expect some difference between the people involved. This is described as being one man and one woman. The picture then becomes intelligible.