There's a saying that goes "you can never really know someone." Even the best judges of character get duped from time to time. In my opinion, it's only okay to have a relationship outside of marriage if both parties are aware of and in agreement with the arrangement, whether it's online or real life. The realization that one is submissive or dominant while their partner is not doesn't make it okay to cheat in my book because any number of excuses are used all the time by cheaters, i.e. my wife got fat after she had kids or my husband has lost interest in me. I'll use myself as an example. My ex-husband and I had different needs. He didn't mind the online stuff because it got me hot. Any online persons I "played" with knew I was still married. However, it wasn't enough for me. I wanted the real deal. He refused to let me go outside of the marriage, even though I gave him permission to do likewise. In the end, we couldn't work out our differences, which included things outside of the bedroom, and I chose to leave rather than stay and do what I knew would happen eventually...which is that I would either cheat on him or be placed back on antidepressants or both....
So to answer your question, in my opinion, you either need the blessing of the person you made vows to or you need to move on when you realize that it will never work. A third option is to stay and be miserable together for the rest of your lives. I don't think it's fair to start a new relationship under false pretences. I would certainly be furious and hurt if I discovered that my dom was still married instead of divorced as I'm sure he would be if he discovered the same about me. On the flip side, if I were the spouse who discovered my partner having an affair for whatever reason, I would also be furious and hurt. I could never trust someone who lied about something like that, and I didn't want to be the person who couldn't be trusted...so...yeah...it's not okay. But that's just my opinion, and what other people do is really none of my business, unless they're doing it to me.