You say you "are talking to" the agent. You don't say you are a client of that agent. The agent therefore has no realty-related ethical obligations to you.
If the agent is a Realtor, then there's the Code of Ethics. But the Code only addresses professional behavior as it relates to real estate, business, and the economy. See http://www.realtor.org/mempolweb.nsf/pages/2011code
Here's an excerpt from the Preamble:
Under all is the land. Upon its wise utilization and widely allocated ownership depend the survival and growth of free institutions and of our civilization. REALTORSÂ® should recognize that the interests of the nation and its citizens require the highest and best use of the land and the widest distribution of land ownership. They require the creation of adequate housing, the building of functioning cities, the development of productive industries and farms, and the preservation of a healthful environment.
Such interests impose obligations beyond those of ordinary commerce. They impose grave social responsibility and a patriotic duty to which REALTORSÂ® should dedicate themselves, and for which they should be diligent in preparing themselves. REALTORSÂ®, therefore, are zealous to maintain and improve the standards of their calling and share with their fellow REALTORSÂ® a common responsibility for its integrity and honor.
In recognition and appreciation of their obligations to clients, customers, the public, and each other, REALTORSÂ® continuously strive to become and remain informed on issues affecting real estate and, as knowledgeable professionals, they willingly share the fruit of their experience and study with others. They identify and take steps, through enforcement of this Code of Ethics and by assisting appropriate regulatory bodies, to eliminate practices which may damage the public or which might discredit or bring dishonor to the real estate profession. REALTORSÂ® having direct personal knowledge of conduct that may violate the Code of Ethics involving misappropriation of client or customer funds or property, willful discrimination, or fraud resulting in substantial economic harm, bring such matters to the attention of the appropriate Board or Association of REALTORSÂ®. (Amended 1/00)
Now, if the agent is a Realtor (which you haven't established) AND if the agent acted improperly as a Realtor (whether or not due to the affair), then there is an ethical issue involved.
An agent having an affair with your husband may be immoral or improper. But that alone does not make it unethical.
Hope that helps.