An open marriage is a "marriage" whereby the spouses have mutually agreed to have other sexual partners. Depending on the "agreement" the other relationships can be heterosexual or homosexual or bisexual, or any or all of the above.
Biblically it is considered a form of adultery, based on the Biblical view of "one man and one woman". In secular culture, though, the term "adultery" is generally limited to a extra-marital sexual relationship by one spouse where the other has not consented to it.
There are, of course, no genuine statistics proving that such an arrangement is any more beneficial, or at least neutral, than traditional marriage which has lasted since the beginning of creation.