On a busy night last week at the Target on Atlantic Avenue in Brooklyn, Ari Nagel, 40, emerged from the men’s bathroom looking a little flushed and quite pleased with himself.

“It’s better when it’s fresh,” he told them.

“It” is Nagel’s semen, and it’s in demand. The 6-foot-2 CUNY Kingsborough math professor has served as a sperm donor for dozens of locals, siring 22 kids over the past 12 years with 18 women of various backgrounds.

For lesbian couples and single ladies looking to have a baby without the expense of going through a sperm bank (which can run in the thousands of dollars), he’s the No. 1 dad.

“This isn’t time-consuming, and I’m doing it anyway,” he says of his hands-on hobby. “It’s very easy for me to do.”

His oldest child, now 12, was conceived with a woman he was in a committed relationship with, but all of his offspring since, he says, have resulted from his donations.

About half the time, he provides his seed the old-fashioned way. Sometimes, a lesbian looking to conceive will have her partner in the bed for moral support while she and Nagel engage in intercourse.

“She’s never slept with a guy before, so the partner’s in bed, holding her hand,” Nagel explains. “Sometimes, it could be a little painful, then after a few times, they’re comfortable to do it on their own.”

Other times, he supplies his goods in a cup, which he prefers.

“I’m not doing it for easy action,” Nagel says. “Isn’t that what Tinder is for?”

He often uses public bathrooms, like those at Target and at Starbucks shops, to procure his samples and hand them off to ovulating women.

“You don’t want to do it in one where people are knocking,” he notes.

He will also offer his services in his home near Downtown Brooklyn, but mama wannabes are often more comfortable meeting in public.

Once a location is chosen, Nagel will go into the bathroom, pleasure himself while watching porn on his iPhone — “You can’t connect to Target Wi-Fi if you’re connecting to a porn site, so I use my cell service,” he says — and ejaculate into an Instead Softcup, a type of menstrual cup.

He then delivers the specimen to the woman, who goes into the ladies’ restroom and inserts it into her cervix.

“I can keep it in for 12 hours,” says Dege, a 40-year-old lesbian from The Bronx who was one of the women meeting Nagel at the Target last week.

Dege, who declined to give her last name, had tried a few times before using Nagel’s sperm, but hadn’t yet conceived.

This time might do the trick. The prolific professor is often successful, which he attributes to a high sperm count: 85 million per milliliter.

“It’s off the charts,” he boasts. “The clinic said they’ve never seen anything like it.”

(The Mayo Clinic says normal sperm density ranges from 15 million to greater than 200 million sperm per milliliter.)

Nagel made his first foray into professional baby-making eight years ago with a friend — a single, straight Jewish woman in her late 30s and living on the Upper West Side.

“I actually tried to fix her up. I had a friend who I thought would be a better match as a sperm donor,” he says. “He got cold feet at the last minute.”

So Nagel went with the woman to the fertility clinic.

Then he helped out two lesbians seeking a donor on Craigslist. Other women have heard about him through friends and Known Donor Registry, a free website for those looking for sperm donors.

‘I’m not doing it for easy action. Isn’t that what Tinder is for? … I just love seeing how happy the moms and kids are. It’s the gift that keeps on giving.’ - Ari Nagel

Women who have used Nagel’s services — which he provides for free — say his good looks, personality and high sperm count are a draw.

“He’s a lot of fun to be around, he loves people, he’s outgoing, and he’s gorgeous,” says Tiffany Harrison, 41, of New Jersey, who with her wife, Yvonne, has a toddler daughter, Zoe, sired by Nagel.

As for his own motivations, the big daddy insists he just likes spreading his seed.

“I just love seeing how happy the moms and kids are . . . That’s why I do this,” he says. “It’s the gift that keeps on giving.”

And Nagel, who grew up in an Orthodox Jewish family with six siblings, says he gets the benefits of having a large brood without the hassle.

“I feel like [I’m] getting all the joy, but also getting a good night’s rest,” he says.

Nagel has a Facebook album of photos of his kids and regularly baby-sits and attends birthday parties and graduations. He has even been present for a handful of deliveries.

“Single women prefer me to their mom,” he claims.

Nagel says his name appears on the birth certificate for just under half of his offspring. Some take his surname, and there’s even an Ari Jr. and two Arias. A few families have used him multiple times.

“A lot of them want another sibling for the [first] child,” he says.

And Nagel’s seed-sowing isn’t a drain on his love life. He doesn’t make a point of mentioning it on dates, but when it comes up, ladies typically don’t mind.

“Never underestimate the desperation of a single woman on the Upper West Side,” he says.

But it’s not all sunshine and babies.

The first five women he worked with successfully sued him for child support, and nearly half of his paycheck is garnished for his offspring.

“I don’t know what’s more surprising: that five sued or that 17 didn’t,” Nagel says. “They were all well aware there was no financial obligation on my part. They all promise in advance they won’t sue.”

Crystal, a Connecticut woman who has two sons, 6 and 7, by Nagel, says she wasn’t aware of any such arrangement.

The 45-year-old mom, who took Nagel to court for child support, says that she was expecting to co-parent with him and that she didn’t know of his plans to father an entire baseball team.

“My kids got left in the dust,” says the woman, who conceived both boys through intercourse. “You can’t co-parent with 20-something kids.”

(The other four who sued could not be reached or had no comment.)

The big daddy says that the angry moms are just hungry for money and that there was no misunderstanding.

“I think they went in with the intention to sue,” he claims.

Still, he hasn’t bothered taking preventative legal measures, because such agreements are rarely binding.

Less-contentious baby-mama drama can begin well before a kid is born.

“Two women can be surging [highly fertile] the same day,” he says. “I have to make a decision. Do I go with the 45-year-old because she’s running out of time and every month makes a difference? Or do I go with the 30-year-old who says, ‘Am I chopped liver?’?”

And it continues as the kids grow.

“Some of the mothers, they have attitudes,” says Shaniece Cromer, a Bronx single mom to an 11-month-old girl, Taraji, whom she conceived with Nagel via in-vitro fertilization. “The jealous moms feel like their kids don’t get enough time with him.”

Nagel’s progeny isn’t limited to the tri-state area. He has kids in Florida, Illinois, Virginia, Connecticut and Israel. Some he sees once a week, some he sees once a year, some he’s never met.

Despite the court cases and child-support payments, Nagel says he has no regrets. He’s open to more kids and says he’s in talks with several women looking to conceive, although he admits he’s getting a bit old for the job.

“Financially, it’s bankrupted me, but I’m still very happy with the way things turned out,” he says. “I got 22 million in the bank — in my kids.”