Is your marriage a happy one? No marriage is happy all the time.
Why do you do it?
Would you leave your spouse for your D/s partner? I wouldn't leave my husband for ANYONE.
Would you meet them in r/l? no
Is is cheating? FOR ME, yes it is cheating. You can cheat with your heart just as much as you can cheat with your body parts. However, this is my personal belief...I'm not saying that all of you out there are cheaters. It is not my place to judge and everyone has different opinons and different defintions of what cheating is.
When does it become cheating? AGAIN..personal belief...when you give your heart to someone else that isn't your spouse...