Why don't you just say their Dad is seeing a married woman and you think that isn't right but Dad has his reasons...I see nothing wrong with letting them know this fact because it is the truth. Just try not to get them to choose sides...

Don't play games with your sons...They are step sons...and will always see and forgive their Dad...blood is thicker afterall.

You stay strong...and in the end, you will look and feel good because it isn't you that ran off with a married person.