Most Christians oppose gay marriage and gay adoption rights because of their scriptures (and the most prevalent Christian theology that's taught by most denominations.) Not only that, but a good portion of them not only oppose those things, they moreover think that if a country were to accept and legalize them, bad things would happen because that would "anger God."
Of course in most civilized countries, especially in Europe, they can't just go to a government representative and say that. It would be simply ignored as a religious dogma that has no bearing on what the government should do. Therefore these Christians have to come up with some alternative, secular reasons, why gays shouldn't get these rights.
This is another form of insincerity by Christians. Everybody knows the real reason why they oppose equal gay marriage rights, but they have to come up with excuses to circumvent those reasons and invent more "plausible" reasons to protest.
This, in turn, causes them to engage in even more insincerity and distortion. The most common excuse they come up with is "the rights of children to have a mother and a father." They completely ignore the studies that have been made on the subject (whose result has been that there's no significant difference between heterosexual and homosexual parents with regard to the wellbeing of a child), and oftentimes they outright fabricate "thousands of studies" that show the opposite. (Don't hold your breath waiting for them to give the thousands of references. They don't exist.)
When you bring forth the most obvious flaw in their argument, they immediately engage in double-think. In other words, if their argument is that "children have the right to a mother and a father", that would mean that single parents should have no rights to their own children (in the same way as homosexual couples should have no rights to get children.) I have had a Christian literally answer to me along the lines of: "What do you mean? Of course they have rights to their own children!" Seemingly this Christian was completely incapable of seeing the connection between the two arguments, which would have shown a big flaw in his.
It seems to me that most Christians are just repeating this argument like a mantra, without even understanding what they are saying.