That's not the point. Your union allows the possibility of new life. No other type of union does. We're not talking about individuals, we're talking about institutions.
If you and your wife have sex, CAN you create a baby?
"Since my wife and I have decided not to have any children, we are not considered married?"
In the past, like it or not, you would be considered married but selfish.
It's your decision, as far as I am concerned. The majority of married couples do produce children. The institution of marriage is big enough and broad enough to include couples who, for various reasons, do not produce children. These couples can include the elderly and infertile couples. It would not be practicle to exclude all heterosexual couples who do not have children from the institution of marriage. The point is that all of these couples are male-female and do not seriously undermine the basic rules of marriage. (The rules also include prohibitions against marrying close relatives and those who are underage. These rules also essentially revolve around procreation.)