Don't know why you got downvoted. My CC was recently compromised, and the "thief" purchased a lot of very odd things all at once which triggered the flag, including online services I had never heard of. And lots of dominos pizza!
Except as many others have pointed out, having an account on the site with a matching credit card number does not offer proof of cheating, nor even of intent.
I'm not very sympathetic to the people making this argument (the "it ruined my life!" crowd), because the reality is that probably 90%+ of all the account holders cheated or intended to cheat while married, without their spouse's approval or knowledge. As far as I'm concerned, if you had an account there, it was up to you to make sure your spouse knew that, and why. And it's your fault if your spouse is unhappy about it or doesn't believe your post hoc explanation. The criminals responsible for the leak need to be punished severely, but the criminal nature of their acts doesn't let you off the hook.
Nevertheless, this all falls well within reasonable doubt for me, so I'm not going to call someone a "cheating spouse" even if 100% certain they had an account. For one, I don't care; what is and not cheating is between spouses. For another, I don't need a libel lawsuit. And last, calling someone a cheater doesn't provide me any benefit anyway.