Hallmark added the cards after California joined Massachusetts as the only U.S. states with legal gay marriage. A handful of other states have recognized same-sex civil unions.
The language inside the cards is neutral, with no mention of wedding or marriage, making them also suitable for a commitment ceremony. Hallmark says the move is a response to consumer demand, not any political pressure.
"It's our goal to be as relevant as possible to as many people as we can," Hallmark spokeswoman Sarah Gronberg Kolell said.
Hallmark's largest competitor, American Greetings Corp., has no plans to enter the market, saying its current offerings are general enough to speak to a lot of different relationships.
Hallmark started offering "coming out" cards last year, and the four designs of same-sex marriage cards are being gradually released this summer and will be widely available by next year. No sales figures were available yet.
"When I have shopped for situations like babies or weddings for gay friends I have good luck in quirky stores," said Kathryn Hamm, president of the Web site gayweddings.com.
"But if you are just in a generic store ... the bride and groom symbol or words are in most cards," she said. "It becomes difficult to find some that are neutral but have some style."
The Williams Institute at the UCLA School of Law estimates that more than 85,000 same-sex couples in the United States have entered into a legal relationship since 1997, when Hawaii started offering some legal benefits to same-sex partners.
It estimates nearly 120,000 more couples will marry in California during the next three years - and that means millions of potential dollars for all sorts of wedding-industry businesses.
Hallmark, known more for its Midwest mores than progressive greetings, has added a wider variety lately. It now offers cards for difficulty getting pregnant or going through rehab.
It pulled a controversial card that featured the word "queer" in the punch line after it was criticized by some customers and gay magazine The Advocate last year. At any given time, Hallmark has 200 different wedding cards on the market, including some catering to interracial or inter-religious marriages and blended families.
The Greeting Card Association, a trade group, says it does not track how many companies provide same-sex cards but believes the number is expanding.
"The fact that you have someone like Hallmark going into that niche shows it's growing and signals a trend," said Barbara Miller, a spokeswoman for the association.
Rob Fortier, an independent card maker who runs his company, Paper Words, out of New York, added same-sex wedding cards to his mix after thinking about what he would want to receive.
"A lot of people think a gay greeting card needs a rainbow on it," Fortier said. "I don't want that."
But for some time, it was difficult to even find the words for what anyone wanted to say, he said.
His first card poked fun at the challenge. On the outside it featured lines that had been scratched out: "Congratulations on being committed!", "Congratulations on being unionized!" and, finally, "Congratulations on being domestically partnered!" The inside wished the couple congratulations on choosing to be together forever.
"It really comes down to language," he said.
John Stark, one of the three founders of Three Way Design in Boston, which makes gay-themed cards for occasions from adoption to weddings, has several new designs sketched out and ready.
But he has hesitated adding more wedding cards to his mix until after the November election, when California voters will decide a constitutional amendment that would again limit marriage to a man and a woman in the state.
"What is scary is to produce a marriage line and then November comes and it's recalled, then we have thousands of dollars of inventory waiting," he said.
The gay-friendly business can be challenging, companies said.
Hamm said although she has found many vendors willing to work with her company, some have asked to be removed from the Web site because of hate mail or some other backlash.
Hallmark says all of its stores can choose whether they want to add the latest offerings.