The gay marriage advocates should be careful about opening this can of worms.
If heteros have to produce children within three years, or face annulment, then homosexual couples have the same requirement. And once annulled, the homosexual couples would have to start the whole process of getting their marriage reinstated only after they have shown due diligence in learning whether they are physically capable of becoming parents.
Or we could just go to "limited" cohabitation contracts, which are renewed upon the payment of an annual fee to the civil authorities, in which children are not involved at all. The gender of the contracting individuals would not be a matter of concern to the civil authorities in the least, but evasion of payment of the annual fee would become a criminally prosecutable offense.
Take care what it is that you wish for.
People aren't getting the point of this, are they?
The point is that it demolishes one of the common arguments against gay marriage - "they can't procreate" - used as an excuse to deny legal recognition of their partnership under the "marriage" umbrella.
This is why it's best to refrain from such specious arguments when debating gay marriage, and stick to either getting the government out of the marriage business entirely, or focusing on the vast problems inherent in changing the meaning of the word "marriage" to include something that the authors of the laws had absolutely no intention of including.