I think there's no hard and fast rule, it depends on all concerned. A friend of mine's DH has a female best friend and it's not an issue for them and they have a rock solid marriage.
But for me, I wouldn't be comfortable because I know DH and he would run the risk of it becoming an emotional affair. It's just the kind of person he is. I know it, he knows it, there's history... So we have certain relationship boundaries we stick to (both of us) and it works for us.