There is nothing wrong with having a bestie as a friend or the opposite sex when single, it could be accepted if it is the same sex, but not opposite because it would create doubt and suspicion in relationships with couples not trusting their partners' besties when it happens to be the opposite sex, but if the same sex, most couples have no issues with it and are okay for their spouse to keep their besties of same sex not opposite sex.
Naturally, humans are jealous beings, and not to mention when it has to do with relationship matters, it would be too obvious to allow such to happen with their partners having the opposite sex as a bestie. Most people are not comfortable with such and would not allow their partners to continue with it, but it depends on both's agreement and understanding.