Aside from singles hooking up, there is speculation that Tinder helps people in supposedly committed relationships cheat. In the first quarter of 2015, research firm GlobalWebIndex used survey data to determine that 42 percent of Tinder users were NOT single, a figure disputed by Tinder [sources: McGrath, McHugh, Olson].
Some non-singles reportedly use Tinder for things other than finding extramarital sex partners, sometimes alongside their single friends or even their own partners, possibly out of curiosity, inclusion or the fun of the game. Couples have apparently been known to use it to find threesome participants (an activity for which competing app 3nder was created).
But we all know that people sometimes cheat on their partners. A survey by YouGov in 2015 found that 21 percent of male respondents and 19 percent of female respondents admitted to cheating, and 7 percent declined to answer the question [source: NPR]. Research has even found that when people have lots of choices of potential mating partners, they are more likely to divorce [sources: Cohen, McKinnish, South]. Tinder is a handy tool for finding people looking for love, sex, or both, which makes it likely that some non-single Tinder users, however many or few there are, use it to find cheating partners.
Like many online platforms before Tinder, it has also been reported that sex workers have used the app to get clients, although many of the apparent prostitutes are fake scam accounts. Illegal activity is not allowed on Tinder (or anywhere else, for that matter, but its very nature). But like cheating, it still happens, and technology helps it along.