I am married but what if I have a secret?
Do you want to tell me?
I'm in a relationship with another married man
It is wrong to be unfaithful to your spouse, and you are deceiving him by having this secret relationship, which could cause a divorce.  You should be open and honest about everything in a marriage, and if you can't end this other relationship, your spouse at least deserves to know.
I think it's okay as long as he doesn't mind.
You need to tell him. Usually people are honest with each other when they are in a marriage.