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 Washington Defense of Marriage Alliance acknowledged on its Web site that the initiative was "absurd" but hoped the idea prompts "discussion about the many misguided assumptions" underlying a state Supreme Court ruling that upheld a ban on same-sex marriage.

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.

The paperwork for the measure was submitted last month. Supporters must gather at least 224,800 signatures by July 6 to put it on the November ballot.

The group said the proposal was aimed at "social conservatives who have long screamed that marriage exists for the sole purpose of procreation."

Cheryl Haskins, executive director of Allies for Marriage and Children, said opponents of same-sex marriage want only to preserve marriage as the union of a man and a woman.

"Some of those unions produce children and some of them don't," she said.