I’m sorry you had the hurtful experience of learning your husband did that. Is it a site he is still active on or was it from a year ago? Since he told you about it then, it makes me think he didn’t feel he was doing something wrong. Have you confronted him about it yet? Giving him the benefit of the doubt, could he have omitted his marital status and been in a singles group for some other purpose than to cheat?

I completely understand about how you will stay with him regardless because you need him. Good for you for not blowing it up only to hurt yourself and rather being wise and careful about how to proceed.