God permitted polygamy in the Old Testament but this only led to sin. Even some of the famous Bible characters, Abraham and David, committed polygamy. The result of this was they encountered problems that they did not wish they had. God’s design is a man to be married to a woman and not to have multiple marriages. Polygamy only leads a married person far from God.
A true Christian will never be a mistress to any married man or woman as they know that God hates people who are ruining a marriage. With that said, a non-believer is the most likely candidate to have a relationship with a married person. This will only cause problems as they will only lead each other to a life of sin.