You have absolutely been cheated on, and betrayed. Just like every BS here.
You have an extra layer of betrayal, because he has lied to you about his sexuality.
While being gay,bi, or whatever, is not a choice, fidelity IS. You do not have to be ok with him having sexual contact with men. AT ALL. He is attracted to women as well. You wouldn't be ok with him having sex with other women,right? This is NO different. You are attracted to men. You choose to be faithful. Being bi, if he is, simply means the pool of attractive people is larger than a straight man's pool,so to speak. Again,fidelity is a choice.
How do you know if he is bi? Time. And his willingness to get into IC and figure out his issues. He should be willing to sign a paper at his IC, that allows you to speak to his therapist, so you know he addressing his infidelity and sexuality.
What he needs to do:
Full transparency. You get full access to everything, all online accounts and his phone. Passwords included.
He sends a NC email to OM. One you approve of. And he stays NC.
He answers all of your questions, without blame or anger, as often as you need to ask.
He gets a full panel of std tests. You go with him to this appointment, so you know the doctor knows he has been with men, as there are certain tests he will need. And the results are sent to you.
He is accountable for his time when he is away from you.
Complete honesty. About all things.
No blameshifting. He owns what he has done.
And anything else you need to feel safe.
What you need to do:
Full panel of std tests
See an attorney to know your rights
IC. This is a trauma. You will need help processing this.
Be kind to yourself
Don't do anything towards reconciliation. Not yet. He needs to do the heavy lifting. You sit back,and watch what he does. Words mean nothing. Watch his actions.
Contact the other man's wife,if there is one. Do not tell your husband. He may warn him,so by the time you speak to her,she will have been told you are crazy and abusive. Also,by not telling him, if he confronts you,you know he is still in contact with OM.
I'm so sorry this happened.