There are two ways heretics defend their views. The first is to read a statement and extrapolate it far more than it was ever intended to be extrapolated. The other method is to not allow for any extrapolation what so ever.
When confronted with a doctrine, one can reasonably ask "where does the Bible say..."? For example, where does the Bible say gay marriage is an oxymoron? But heretics add three little words that make even the most easily defended doctrine indefensible: Where does the Bible say that in those words?
By requiring exact words, the heretic can brush aside any explicit statements that clearly show the doctrine we are trying to defend. Sure, the Bible says it, but it doesn't say it in those words, so it doesn't really say it, right?
Let's take the most absurd statement an atheist has ever made for a moment. I was once told that Jesus used to take young boys into His house and rape them. Now, where does the Bible say "Jesus never raped young boys" in those exact words?
I seriously don't recommend entertaining anyone foolish enough to actually require you to defend the position that Jesus wasn't a violent pedophile. If they're going to say stupid things like that, cut the conversation there and leave them to stew in their own insanity. Only God can help such a moron. But for sake of demonstration, let us note that the Bible absolutely never says this in any kind of exact words. In fact, as far as I'm aware, it doesn't comment on Jesus' sex life at all. What it does say is that Jesus knew no sin. If Jesus knew no sin, He couldn't have raped a young boy, nor indeed had any sexual relations at all, as He never took a wife.
Now you see the absurdity of the exact words criteria. You can come up with any number of insane beliefs and defend them by asking "where does the Bible say I'm wrong in those exact words?"
So, let's slice the "in those words" from the end of the question and just ask "where does the Bible say gay marriage is an oxymoron?" Well, in Matthew 19:4-6, Jesus reminds His audience that Genesis describes the origin of marriage. Specifically, God made one man and one woman and designed them to be "one flesh" (i.e. they physically complete each other, as each gender has the parts the other lacks, enabling them to reproduce). In other words, those who say "God made Adam and Eve, not Adam and Steve" have an excellent point. Jesus gave us a standard and authoritative definition of marriage, and in His explanation, He made as good a refutation of gay "marriage" as He gave of polygamy and, as He was specifically addressing, divorce.
But you don't have to stop there. If marriage is defined as when the couple becomes "one flesh", then obviously attempting to become "one flesh" with the same sex is out of the question. Why? Because a gay couple can never become "one flesh".* Furthermore, we have even more explicit statements that homosexuality is unnatural (Romans 1:26-27), an abomination (Leviticus 18:22), and that it keeps men out of Heaven (1 Corinthians 6:9-10).
We see, then, that there is no real way to crowbar gay "marriage" into the Bible. Homosexuality is universally condemned in the Bible. It does not have to say "gay marriage is an oxymoron" in order for that to be a 100% true, Biblical statement.
*At this point, the objection of infertile couples is often raised. This is easily refuted by the fact that although the specific couple cannot reproduce, they are the right type to reproduce. You can compare this to a phone and a charger. Just because the charger is broken does not mean you can use another phone as a charger. It just doesn't work. Chargers were designed for phones, men were designed for women.