This problem can be solved by the simple method of telling your wife that you have such an account.
And that would help how?
Why would it matter?
People are illogical. They would cast a huge black shadow around other people because they greatly exaggerate the importance of small deeds from the past.
I once created an account on a dating site. It was for both single and married people. I was single and looked for other single people. Ironically, it's how I met my wife. We're married now and have two boys. As far as I know, that account still exists. Obviously she knows I have created it, that's how we met, doh.
If you don't care whether your friend violates the trust of the person he's supposed to be closest to, that's your decision. But most people would consider that an extremely low standard. Would you care if he lied to you?
(If his wife knows about it and is okay with it, that's different. But it doesn't sound like that's what you're talking about.)
No, I don't care. He's master over his own life and unlike others I don't impose my morality over his. Again, I don't particularly like that trait but won't "unfriend" him because of it. Also, I don't give a flying fuck what other people consider this as standards because I believe in freedom of choice. And if he lies to me, in a way that it affects me, then I'll get to weigh in all arguments and decide whether he's still my friend or not.
Generally speaking, I value feelings more than sex, and to me, they're not entwined. One could have casual sex with other people while still loving their wife or husband and there's no overlap. It's culture-related I guess. Middle East is on one extreme, parts of Western Europe are leaning towards the other extreme and U S of A is somewhere in between.