Marriage is a civil and a religious privilege.
Proof that its a privilege is demonstrated by the fact the union can be dissolved if the privilege is not exercised properly or has been abused, by the act of divorce.
Thus, marriage can be withdrawn from those who have freely entered into it. In contrast, a right can never be suspended or revoked.
Huh? The right to vote and the right to bear arms, for example, are routinely taken away from convicted felons (even after they finish their prison sentences).