Pursuing intimate relationship with married woman with whom I am in love--what rule applies, if any?

If there some "rule" for modern, progressive people to follow in this situation, what is it? I do see an ethical issue in the fact that if the affair goes forward, the woman I love will probably have to tell some lies to her husband to keep him unaware of the affair. And I do see a possible ethical issue in the fact that when she married her husband she probably made a promise to him, implicitly or explicitly, that she would not have with other men. I do think it is usually important to be faithful to one's promises, and to be loyal to close family and friends. Fidelity, trustworthiness and values do matter to me. I hate the cynical idea that we are all dead in the long wrong, so nothing really matters, so we ought to just do what pleases ourselves before we die. But it seems too harsh to follow a rule that married people can NEVER date and have outside of their marriage. That would mean that every married person is forever ENSLAVED to their spouse. It seems too harsh to have a rule that you have to divorce your spouse before you can date or have with other people. I do want to be a person of integrity. But I also want to give love to and receive love from this wonderful woman. If I want to have the affair, and she wants to, what's the harm? (By the way, religious commandments from olden times mean nothing to me, since I think they were crafted by men, not God, and they don't apply to modern times.).