In Playboy's September issue, Hefner, 86, says the fight for gay marriage is "a fight for all our rights", reported Us magazine.
"Without it, we will turn back the sexual revolution and return to an earlier, puritanical time. Today, in every instance of sexual rights falling under attack, you'll find legislation forced into place by people who practice discrimination disguised as religious freedom. Their goal is to dehumanise everyone's sexuality and reduce us to using sex for the sole purpose of perpetuating our species. To that end, they will criminalise your entire sex life," Hefner wrote.
"[Conservatives] assault the right of gays, whether by denying them the right to marry or, as in Kansas, by attempting to empower landlords, business owners and employers to discriminate against gays on religious grounds... This is a religious nation, but it is also a secular one," he added.
"No one should have to subjugate their religious freedom, and no one should have their personal freedoms infringed. This is America and we must protect the rights of all Americans."
Hefner is pleased with the response his letter has already received. He tweeted Wednesday, "My editorial titled 'Sexual Freedom' on gay rights in the September issue of Playboy is generating a lot of positive response."