During the late 1950's a popular activity for adults, married and unmarried was swinging. This was not getting on the porch, and swaying back and forth, in a lovely breeze, unless the people were naked, and getting it on.
Into the 1960's a type of sexual freedom emerged, where couples would agree to an "open marriage," or open relationship. This meant couple would openly engage in sexual activity with the knowledge of the partner.
This like any other idea, had its pitfalls, but for those who see relationships and human interactions as free, do go with the open relationship plan. Some want to know, others, not so much.
It is okay, if the "other woman" is okay, and has no expectations in the relationship.