Hi, I'd like to help you with your question.
From what you said, it sounds like your husband had a relationship with this woman, possibly had a child with her, then continued the relationship off and on, even while married to you.
There is no way to tell if he is being truthful at this point. You can ask him to get a paternity test, but if he is not willing and he is not paying child support, there is no real motivation except his willingness to do it for you, which he should for your marriage and the trust issues this has created.
His expectation that you treat this relationship as if he never cheated on you is unrealistic. There is no way for you to pretend the situation did not exist. He may chose to do this, but to ask you to do it as well says he is not dealing with the situation as he should be.
I highly recommend you both seek counseling together. You need the support, and you also need to talk this out with a neutral person so you can be sure that the topic is equal and fair. You also need to work on trust. If your husband is just informing you of this now and had a whole history with this person and possibly a child, you needed to know about it. Since you did not, this becomes a serious trust issue and trust is vital for a relationship to continue.
He also needs to reassure you that he will discontinue contact with this woman. Since the child is an adult, he no longer needs to have contact with the mother. He can develop a relationship with his daughter, but you must be included and he must acknowledge you as a priority in his life. Although that does not mean you get included in every contact he has with his daughter, you need to be informed of it and kept abreast of the relationship. This is also to help rebuild your trust with him.
I hope this has helped you,
I agree with you. When you husband had this child, he needed to take responsibility for him/her and also let you know about it. This also includes your children. They do have the right to speak with their father about the situation and chose whether or not they want contact with their half sibling.
It sounds as if your husband wants to control the situation and have everyone agree with him. However, even if you agreed to do what he wants, you would still have your feelings to deal with about the situation. He may be able to block out the child and distance himself from the situation, but he cannot expect others to do the same.
Again, it is also about the trust issue in your marriage. You are an equal partner and need equal say in how this works out. That is why I think counseling would be very effective.
Given that he has already been dishonest and is not facing what he did by admitting it, chances are probably not that good he will be honest in the future.
I think it would be advisable for you to seek counseling with him. That is the only way you are going to get the answer you are looking for. I can only give you an idea of what I think based on what you have told me. You need someone who can sit down with you, go over the history of this problem and get a detailed idea of where this problem stands now. The counselor also can evalutate your husband to see where he stands with this as well.