BOSTON ­ Now I got it. After hours spent poring over Washington state's Supreme Court decision upholding the ban on same-sex marriage, I've finally figured it out.

The court wasn't ruling against just same-sex marriage. It was ruling in favor of "procreationist marriage.''

This is the heart of Justice Barbara Madsen's opinion: "Limiting marriage to opposite-sex couples furthers procreation, essential to survival of the human race, and furthers the well-being of children by encouraging families where children are reared in homes headed by the children's biological parents.'' In short, the state's wedding bells are ringing for procreators.

Well if that's true, isn't it time for the legislatures in Washington and in New York, which issued a similar ruling against same-sex marriage this summer, to follow their own logic? If marriage is for procreation, shouldn't they refuse to wed anyone past menopause? Shouldn't they withhold a license, let alone blessings and benefits, from anyone who is infertile? As for those who choose to be childless? Nothing borrowed or blue for them. Indeed the state could offer young couples licenses with sunset clauses. After five years they have to put up (kids) or split up.

Of course the states' other interest is in families "headed by the children's biological parents.'' Why then give licenses to the couples who are raising 1.5 million adopted children? We can ban those blended families like, say, the Brady Bunch. And surely we should release partners from their vows upon delivery of their offspring to the nearest college campus.

This is where the courts' reasoning leads us, and I use the word "reasoning'' loosely. If anything, these two decisions are proof that the courts and the country are running out of reasons for treating straight and gay citizens differently.

Since the landmark Supreme Court ruling in Lawrence v. Texas in 2003, gay sex is no longer a crime. Today, if some straight couples cannot or do not procreate, some gay couples do, using all the old and new technologies. Fertility clinics don't ban gays. They aren't the slam-dunk losers in divorce custody fights. Even Arkansas has just ruled that gay couples can become foster parents. And New York and Washington, the very states now refusing to let gays marry, have supported gay adoption.

Against this evolving backdrop, the courts had to reach pretty far to find some explanation for banning gay marriage other than old-fashioned discrimination. Even so ­ as Justice Mary Fairhurst wrote in her Washington dissent ­ neither court actually explained why "giving same-sex couples the same right that opposite-sex couples enjoy (would) injure the state's interest in procreation and healthy child rearing." After all, as Chief Judge Judith Kaye of New York wrote in her dissent, "There are enough marriage licenses to go around. ... No one rationally decides to have children because gays and lesbians are excluded from marriage.''

I am a citizen of Massachusetts where gay people have been getting married for two years without the sky falling. (The ceiling on the Big Dig has fallen, but that's another story.) The furor over the decision here produced a backlash that has scared a lot of judges straight. The current decisions reek of that anxiety.

These judges seem ready to bow to any legislation on this hot-button subject that isn't certifiably nuts. For example, the American Academy of Pediatrics reports that "There is ample evidence to show that children raised by same-gender parents fare as well as those raised by heterosexual parents.'' The Washington court still determined that "the legislature was entitled to believe'' the opposite. The legislature's entitlement overruled gay entitlement to marry.

The backlash against gay marriage has produced strong passions and weak arguments. It's no longer enough to state in court that marriage has always been for straight couples, ergo it should be only for straight couples. This time the courts ended up arguing on procreationist grounds, pretty shaky legal terrain.

"It is the exclusive and permanent commitment of the marriage partners to one another, not the begetting of children, that is the sine qua non of civil marriage,'' wrote Chief Justice Margaret Marshall in the Massachusetts decision that extended marital rights to gays and brought conservative wrath down on her head.

Marshall has been demonized as an "activist judge'' ­ a label pinned on the author of any ruling you dislike. Now, in an anxious attempt to put their courts into neutral, judges in Washington and New York have thrown logic into reverse.