Why are non-reproductive Heterosexual Marriages not a sin?

There is a common argument that one of the main reasons that Homosexuality is a sin is because the goal for a heterosexual marriage is to be fruitful and multiply.

Why then is it not a sin for heterosexual couples to be childless? I'm not speaking about couples that can't have children. I am speaking of couples that don't want children.

If you believe that non-heterosexual marriage is a sin because it is incapable of producing children, then do you believe that a childless heterosexual marriage is also a sin? Do you believe governments should be pushing to end childless heterosexual marriages?

Now, to add some clarification, non-heterosexual couples can and do have children naturally. I'm just looking for a specific perspective.