No, I absolutely would not hold ANY respect for them.
You are in a relationship you dislike enough to betray your partner? Fine, but I do not understand why these people can't have the balls to at least break up with or let the other partner know before pursuing a mistress.
I wouldn't hold respect for them. If they cheated, it would show their true colors and how in the end they didn't care all that much for their partner in the first place. I wouldn't take a chance in trying to be in a relationship with them again. No way.