I don’t think that the notion of limiting marriage to couples that are not of the same gender is exclusively a Christian concept. Until fairly recently, I don’t think that government recognition of same-sex marriage has been common even among jurisdictions that are not predominantly Christian. And, even today, it is hardly the case that same-sex marriage is forbidden only in countries with a majority Christian population.
I don’t think that the notion of limiting marriage to couples that are not of the same gender is exclusively a Christian concept. Until fairly recently, I don’t think that government recognition of same-sex marriage has been common even among jurisdictions that are not predominantly Christian. And, even today, it is hardly the case that same-sex marriage is forbidden only in countries with a majority Christian population.