You might care if you were married and using OKCupid to find a girlfriend.
You may say that getting exposed for trying to have an affair is a good thing, but that's a still a reason why someone may care how secure their OKCupid activity is.
Certain sexual behaviours are outlawed in certain nations. And may result in death or long incarceration times.
In other nations, it may not be strictly illegal, but is more than enough information that, if publicly released, would result in death threats and other social pressures.