I'm opposed to gay marriage, but I've never cared for the argument that it should be banned because it cannot produce children. By the same principle, straight couples that cannot have children (e.g. the elderly or otherwise sterile) should likewise not be permitted to marry. Now gay marriage activists have acted on this rhetorical flaw:
Proponents of same-sex marriage have introduced a ballot measure that would require heterosexual couples to have a child within three years or have their marriages annulled. . . .
The measure would require couples to prove they can have children to get a marriage license. Couples who do not have children within three years could have their marriages annulled.
All other marriages would be defined as "unrecognized," making those couples ineligible for marriage benefits.
UPDATE: Joe Carter has a rather draconian response:
A number of factors could prevent a married couple from having a child within three years (e.g., what if the child is stillborn?) so it would be unfair to penalize them for something that is beyond their control. Instead, a more reasonable criteria should be established that is based on actions that are solely within their power. For example, all couples who wish to marry--both gay and straight--must be willing and able to engage in "marital acts", acts that are reproductive in type. To paraphrase the WA-DOMA, those couples who cannot or will not engage in marital acts that are reproductive in type should equally be barred from marriage.
So if you're disabled or too old to still engage in "marital acts", then you shouldn't be able to marry, by this reasoning