I agree that having an affair is a sign of problems in a marriage, but what if you've told your partner that you're tempted to have an affair and your partner acts as if that is ridiculous and seems to push you to spend time with the person that you are considering having the affair with...all the time saying that marriage is built on trust and showing no signs of concern about my feelings for this other person? I have been very honest with my husband about my feelings and the conversations that I've had with the other person. My partner has a very uncaring attitude and refuses marriage counseling.
At the End of My Rope
When we're "at the end of our rope" we become desperate. We'll do anything to survive. This is your situation. Your husband is either afraid to deal with the fact that you're very dissatisfied or he unconsciously wants you out of the marriage. He's hiding behind platitudes of marital trust while refusing to deal with the problems in your marriage.
It's clear that you want your marriage, not the affair. I say that because if someone wants to commit adultery he or she does so in secrecy. Telling him what you intend to do is like knocking him over the head with that proverbial sledgehammer and then hoping that he'll react. It doesn't sound like a threat of infidelity is strong enough medicine.
So now you're faced with a difficult moral dilemma. It will be easy to justify having the affair. You've made a serious effort to confront your husband and suggested marital therapy -- all to no avail. Yet, I don't believe you really want to be unfaithful. The fact that you are warning your husband and trying to get him into counseling indicates that infidelity goes against your moral and ethical beliefs.
I suggest you write a letter to your husband and begin by telling him that you want the marriage to work. If you love him, tell him so. Then tell him what is bothering you about the marriage. Try not to blame him but to articulate your feelings of hurt, loneliness and anger. Finish up by saying that you would also like to hear how he feels and that you're committed to making the relationship succeed. Suggest counseling again as a way of helping things improve.
If you've done all this and he still doesn't respond, then I would suggest that you speak to a counselor who can help you sort out your relationship. Based on what you've written, I'm not certain that even an affair will get your husband's attention. Certainly, he will be hurt and angry, but will he begin the real work it takes to create a relationship?