They are two seperate issues.
My point is that what that arguement is about isn't targetted at why homosexual marriage shouldn't be allowed - it's arguing that people who are unable to procreate should not be allowed to marry, which doesn't just include homosexuals. If that is the stance, then why arn't people trying to revoke the marriage of couples who can't have children, yet are heterosexual?
It also brings up a lot of other questions. Would that opinion extend to people who can have children, but choose not to? Does this mean that for a marriage to be valid and wanted, the State mandates they must have children for their marriage to be of worth to the State? Is a marriage viable if the potential for fertility is there? If so, do we allow lesbians to marry as they can recieve IVF or, in other cases, even actually have sex with someone so they can have a child to be brought up by their partner? Does this count subfertility routes to children, such as a gay couple persuing surrogacy?
It's an arguement that raises a lot more questions than it answers. It also doesn't address why homosexuals shouldn't be married. It argues that marriage is a union which requires the potential to procreate for it to be valid, unless you're heterosexual, then you can be married and not reproduce and you're able to remain married while not intending to have children.