"An island can trap a man," he crooned.
"The boat I'm waiting for hasn't come."
The Communist Youth League wants to be that boat. Or to bring him a boat, ideally with a wife in it. What they don't want is for Cai, or tens of millions of other young people, to stay single.
For most of Chinese history, finding Cai a wife would have been a family project. Although Mao banned arranged marriages in 1950, they endured alongside work unit matchmakers until the 1980s, when the economy and the marriage market started to open up.
Forty years into China's economic transformation, dating is a free-for-all of pickups, hookups and setups that give young people looking to marry more choice than ever before. (This does not apply to everyone: Same-sex marriage is banned in China.)
But China's leaders are not fans of free-for-all markets. Last year, they decided there was some correction to do.
Thanks to the one-child policy and a preference for sons, China has a surplus of men. The number of unmarried men between ages 35 and 59 will reach 15 million in 2020, according to one Chinese estimate.
Now it has settled on a more robust market intervention: mass matchmaking.
In Zhejiang, a prosperous province in southern China, an estimated 100,000 young people attended Communist Youth League dating events last year, the group says.
And so it was that on a Sunday afternoon, a couple hundred singles gathered on an artificial beach set in the hills outside Hangzhou, and Cai seized the moment — and the mic.
"You can't find a girlfriend sitting at home," he said.
The man in charge of getting Hangzhou singles out of the house is Wang Huiqiu, a 35-year-old cadre from the Zhejiang Communist Youth League's propaganda department.
Last spring, the league's top brass announced that the organization would henceforth fight "the marriage problem" by helping young people develop the "correct attitude" toward finding a spouse.
Ever since, Wang, who is married, has spent his weeks setting up dating events and his weekends supervising them, a mission he pursues with a sort of nouveau Marxist zeal.
Wang sees matchmaking as a natural fit for a mass organization. Sure, private dating apps are popular, but few have the reach of the Communist Youth League, which has tens of millions of members, he argued.
He urged young people to be skeptical of private apps where scammers abound and prices are high. "A fully commercialized market can be problematic," he said.
Although Wang would never say it, the once-powerful Youth League has been sidelined under President Xi Jinping. Mass matchmaking seems to be a bid to stay relevant — while serving the party, of course.
Wang framed it in economic terms: "The demand for love and marriage is inelastic."
"Of course," he added, "if you meet your partner through the Communist Youth League, you will naturally feel closer to us."
To nurture that closeness, the Zhejiang branch built "a public-interest dating platform," Qin Qing Lian, that is part "swipe right," part "socialism with Chinese characteristics."
After signing up and getting vetted by the local public security bureau, users can browse thousands of profiles.
They can also read dating advice, or "lessons on romantic relations," courtesy of the Communist Youth League. The lesson — singular — is that you should get married as soon as humanly possible.
Men are advised to become providers, women to find a provider, fast. "As long as he's willing to spend money on the woman he loves, he's the right one to marry," one tip says.
Although it is young men who are at high risk of staying permanently single, the focus is on changing women, not men.
On his way to the opening of "Hangzhou Love," Wang worked his phone while mulling ways to get female comrades to marry before they get, as he put it, "picky."
Women are looking for the "three highs: high education, high salary and height," he said, but that's not easy to find.
"As women grow older and their social position rises, they will want better men," he said. "If the Communist Youth League helps women from an earlier stage, it will be easier for them. As early as possible, ideally 25."
When Wang arrived at the venue, the crowd was seated at long tables facing a huge stage. Attendees checked messages and took selfies while Katy Perry, who was recently prevented from entering China, blasted from the speakers.
Seated in the front row alongside Cai were a 25-year old man, Cao Bin, and a 29-year-old woman, Zha Wei.
Cao came to the event because it was co-sponsored by his favorite radio station; he didn't seem concerned about finding a wife.
A recent basketball injury got him thinking that it would be nice to have a girlfriend, though. "I want somebody to take care of me," he said.
He spent the day trying out lines: "Why won't you look me in the eyes?" he asked one young woman. "Is it because I'm too handsome?"
When she ignored him, he asked, "What, am I too outgoing?"
One seat over, Zha was taking things more seriously.
Zha didn't have much confidence in the Youth League — "Are they professional matchmakers? No, they're not," she said — but she came to maximize her man-meeting options.
At 29, she sees herself as having an "age problem" and feels intense pressure to find a husband and have children. "I want my family to see that I'm trying to participate in events like this and reach out to people, so that they know I'm really trying," she said.
The problem is that the things she's proud of, including her master's degree in economics and her experience abroad, seem to hurt, not help, her at events like this. "Men all want 'girly' girls," she said.
Cai, the crooner, did not dispute that women are under more pressure. "Men don't age as quickly as women," he said, as if it were a scientific fact.
He likes that Youth League events draw white-collar workers, but said career success is not the first thing he looks for in a woman. For him, it is "tenderness" that counts.
In the end, "Hangzhou Love" was a lot like mass dating events elsewhere — that is, awkward.
Cao Bin's lines fell flat. Zha Wei didn't get a date. And for Cai's act of courage — unprompted karaoke — he was not rewarded with a number, let alone true love.
But despite the odds of singledom, he was optimistic. Asked about his prospects, he waxed poetic about intertwined hearts, then launched into an actual Tang Dynasty love poem:
I don't have wings to fly with you,
Our hearts as one, you hear my inner call.
He is still single.
Shirley Feng contributed to this report.