There's a big difference between swimming/chatting on MN and having sex with an OM:
Nobody promises when they get married not to go swimming with other people or chat to other people.
OP, you promised when you got married that you'd never have sex with anyone else. You didn't have to get married, if you wanted to have sex with other people - many don't marry for exactly that reason - but you CHOSE to get married.
So basically if you decide to have sex with someone else you are retracting the promise you made. You are saying you've changed your mind. Your husband has the right to know that, and decide if he wants to stay in the "marriage" without that promise, or not. He may prefer to go find someone else who IS willing to be faithful.
It's completely understandable to find you sometimes feel the urge to have sex with someone else. But you made a decision years ago to give up the ability to act on that desire, in return for the security, stability, and all the other benefits of marriage. If you've changed your mind, and you want to have the sex but not the stability, then fine. But tell your DH that. You can't have both.
As for covering your tracks - any marriage in which you're constantly lying to your DH is not a real marriage, IMO.