Two years after the launch of dating app 3nder, Match Group, the owner of red-hot dating app Tinder, is suing for trademark infringement.

3nder has been called the “Tinder for threesomes” since its launch, and the company has long embraced that marketing. (Its official tagline: “Date kinky, curious, and open-minded couples and singles.”) So, one might ask Match Group: Why now?

Dimo Trifonov, 3nder founder and CEO, has a guess.

“They did not care about us before,“ he says. "Their true competitor is someone in their league — size-wise, budget-wise, revenue-wise. But now we are profitable, and we are growing organically without any marketing. Perhaps that is why they care now.”

Trifonov says 3nder, which is based in London, has only spent ￡5,000 ($7,241) on marketing. He says revenue is up nearly 1,000% in the last year and the user base has grown 30% in that time.

This legal action actually began last December, when Match Group (MTCH) lawyers sent 3nder a “letter before claim.” The letter, obtained by Yahoo Finance, accuses 3nder of using an “infringing mark,” with both a name and a logo supposedly meant to remind consumers of Tinder.

That letter gets quite specific: It cites an October 2015 New York Post story on 3nder that states, “The name can be pronounced ‘Thrinder’ to rhyme with ‘Tinder,’ according to its founder.”

3nder now says the name can be pronounced that way, but it is “optional and not an official pronunciation.”

In a press release Monday, 3nder be labors the point: “One of the fun parts of 3nder is the debate on how to say our name. Everyone seems to have their own interpretation, and true to our open-minded values, we embrace all the various pronunciations!”

So, how does Trifonov himself pronounce it? “Three-ender.”

The reason for any prior confusion, he says, is his accent: “I am Bulgarian, so my accent is strong, so that may have been the confusion. But whatever the confusion is, it’s not fair because it doesn’t matter what the names are; the brands are distinct. We don’t want to have anything in common with Tinder.”

3nder said as much in a response to Tinder in January, in which Trifonov laid out the case for why the two apps don’t even compete with each other. Tinder didn’t buy it. On May 10, Match Group officially sued 3nder in the U.K. for trademark infringement.

3nder has quite the origin story.

“My girlfriend had feelings for another girl, and she was worried about it,” Trifonov says, “so she wrote me a letter. And I got really touched by the whole thing.”

He and his girlfriend decided to stay together and look around online for a third partner, but, he jokes, “Online was a really bad place or this. So I said, ‘Let’s make an app for this.’ And it went viral right away, and I said, ‘See, there are so many people like us.’”

If you ask Trifonov, Match Group vs 3nder is David vs. Goliath. “Tinder is so big, and the problem is, if someone is controlling a huge piece of the market, they decide what the norms are,” he says. “They say, Tinder is only for single people. That’s not true or fair. I can date as many people as I want, or if I am in a relationship and my girlfriend and we want to date together, we can do that. If you are in a small town and you are the only polysexual, you might think there is something wrong with you. We are against labels.”

To rally the troops of 3nder supporters, the company is asking all of its users to tweet the hashtag #TinderSuckMySocks. Why that hashtag, exactly? Trifonov has been so busy and stressed over the lawsuit that he hasn’t been able to do laundry, so his socks are dirty. He told Yahoo Finance on Monday that he sent a pair of his dirty socks to Match Group headquarters in Dallas, Tx., and wants 3nder users to do the same.

Legally speaking, the question in a trademark infringement suit is whether a typical consumer might reasonably confuse the offending mark with the plaintiff’s mark. Might users of dating apps confuse 3nder with Tinder?

“That’s impossible,” Trifonov says, “unless you’re blind.”

Tinder and Match Group had no comment on the lawsuit.

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Daniel Roberts is a writer at Yahoo Finance, covering sports business and technology. Follow him on Twitter at @readDanwrite.