As others have said, I think you DO know what you need to do really. Not only do you deserve to have better- but you know better, too. This may sound harsh- but can you really justify the fact that you're stepping into someone else's marriage and basically happy to steal another woman's husband? I don't how she feels about all this- whether she's a big part of their marriage problem, whether they have mutual problems or or whether she acually loves this guy and doesn't suspect a thing- but whichever it is you don't really have any business helping to destroy that marriage and any chance they have of working things out. Hon- it's so easy to say, I know, and so hard to do- but you KNOW that there is no way you can possibly look at it and make it right.
Do you have anyone you trust who you can speak to honestly and who could be a support for you if you find the courage to do the right thing and break off this destructive and deceitful relationship? This sounds very judgmental, I know- but I do understand that your feelings must be going through the wringer and what a battle it must be.