I support the marriage of any consenting adult couple that loves each other. I support the marriage of mixed-gender couples and the marriage of same-gender couples. You don't have to oppose one to support the other.
Allowing the marriage of same-gender couples does no harm to the marriages of mixed-gender couples. But disallowing the marriage of same-gender couples does do harm to them, a harm that is unnecessary. There are same-gender couples who have children, and disallowing them to be married does harm to those children.