The Left passionately supports the most remarkable and radical change in modern social history — the redefinition of marriage from male-female to include male-male and female-female.

Marriage is the building block of society; changing its nature will therefore change society. By allowing same-sex marriage, we are effectively saying that sex (now called “gender”) no longer matters for society’s most important institution; thus, it no longer matters in general. Men and women as distinct entities no longer have significance — which is exactly what the cultural Left and the gay-rights movement advocate, even though the vast majority of Americans who support same-sex marriage do not realize that this is what they are supporting. Most Americans who support same-sex marriage feel (and “feel” is the crucial verb here, as the change to same-sex marriage is much more felt than thought through) that gays should have the right to marry a member of their own sex. It is perceived as unfair to gays that they cannot do so. And that is true. It is unfair to gays.

But the price paid for eliminating this unfairness is enormous: It is the end of marriage as every society has known it. It is also the end of any significance to gender. Men and women are now declared interchangeable. That is why, as I noted in a recent column, the “T” has been added to “GLB” — that is, “Transgendered” has been added to “Gay, Lesbian, and Bisexual.” “T” does not represent transsexuals — people who choose to change their sex. No one is arguing against such people. “Transgendered” refers to people who are members of one sex and who wish publicly to act as if they are members of the other sex, e.g., men wearing women’s clothing in public. The transgendered who publicly act out are living the cultural Left’s primary agenda: rendering gender insignificant. Your sex is what you feel it is; and if you feel both, you are both. Gender doesn’t matter.

That is why Judge Walker and his supporters dismiss the argument that, all things being equal, it is better for children to be raised by a married man and woman than by two men or two women. If Judge Walker or GLBT activists and their supporters admitted that children need a mother and father, they would be affirming that there is great significance to the differences between men and women.

They reject that. Instead, they and Judge Walker offer studies that purport to prove that it makes no difference whether or not a child has parents of both sexes. These academic studies are as unserious as all those studies of a generation ago that “proved” that boys do not prefer to play with trucks and soldiers but would be just as happy to play with dolls and tea sets, and that girls do not prefer dolls and tea sets but would be just as happy to play with trucks and soldiers.

These newer “studies” of same-sex parents are as valid as the earlier propaganda presented in the guise of scientific studies. Like the boy-girl studies, these were conducted by academics with agendas: to deny male-female differences and to promote same-sex marriage. That many Americans believe these studies — studies that are in any case based on a small number of same-sex couples raising a small number of children, during a short amount of time (a couple of decades), based on the researchers’ own notions of what a healthy and successful young person is — only proves how effectively colleges and graduate schools have succeeded in teaching a generation of Americans not to think critically but to accept “studies” in place of common sense.

Ask anyone who supports same-sex marriage this: Do you believe that a mother has something unique to give to a child that no father can give, and that a father has something unique to give a child that no mother can give?

One has to assume that most people — including supporters of same-sex marriage — would respond in the affirmative. How, then, can they support same-sex marriage? The Left’s trinity — compassion, fairness, and equality — is one reason. And “studies” and “facts” are another.

That is exactly how so many college graduates came to believe that boys would be happy with tea sets and girls would be happy with trucks — compassion, fairness, equality, and “studies.” That is also how many Americans, including a judge who overturned a state’s constitutional amendment, have come to believe that never having a mother or never having a father makes absolutely no difference to a child.

And if mothers and fathers are interchangeable, men as men and women as women lose their significance.

— Dennis Prager is a nationally syndicated radio talk-show host and columnist. He may be contacted through his website, dennisprager.com.