Is the unmarried mistress in an affair with a married man doing something wrong?

Joshua, who is married, hits on Catherine, who is single. Now, Catherine knows Joshua is married but gets involved in an affair with him anyway.

Should Catherine be accorded any blame? She is not the one who has committed anything to this Joshua’s wife. It is Joshua that is committed to his wife. Secondly, Catherine owes this married woman nothing, Joshua has the duty/responsibility to protect her and not cheat on her. Obviously we all have our own moral compass and morality is subjective. When it comes to harm, she is not really harming this woman, the husband is. She is simply the means.

Thoughts? Comments?
No I am not having an affair.