It's not so much about the friendship as it is about your personal character and how you handle the friendship, and who you are friends with. I used to throw the baby out with the bathwater and say, "no way can you have any friends of the opposite-sex when you are married ... you will end up having an affair with them!" But that is the same as what some of my religious non-drinker friends say about alcohol, they say that, "if you drink it leads to drunkenness, lewd behaviors and orgies." (That's actually a quote! I know, ridiculous.)
1) The friend should preferable be a friend of both.
This just means that to have a friend that is only your friend could be challenging. Because when they are a friend of both, they will have a respect, hopefully, for the marriage not just the individual. Make sense? There are always exceptions to rules of course, so don't get too hung up on the rule.
2) No secrets.
It should be a totally open friendship. No hiding anything about the friendship from your partner. If you are hiding the friendship, that is a warning sign for sure. There shouldn't be anything to hide ... especially if it's all above board.
3) No confiding.
I have a couple female friends, one in particular I am extremely close with and have been friends with for nearly 12 years and I have never "confided" in her. I haven't told her anything I haven't already talked about with my wife.
4) Never talk bad about your spouse with them.
My rule on this one applies to same-sex friends too. I know it's a bit extreme, but I do not find there to be any benefit in spouse bashing. So I just don't do it. If I have issues with my wife I talk to her about it, or a counselor who will give me sound wisdom. I believe this rule to be extremely valuable to the marriage if followed.
5) Limit Alcohol use when with your friend.
Alcohol, the great elixir. It removes inhibitions as well as morality, in my experience. In other words, if you aren't all that attracted to this friend, you will be after a couple glasses of wine, so be careful. Let's just be honest here, many of the stories that start with, "Well I had a few beers then ..." and they proceed to tell the "bad" story.
6) The character of your friend.
The character of the friend you have makes all the difference in the world. If he/she is a person of character with a strong moral compass it will surely work. Because even if they were attracted to you they wouldn't cross lines because of their strong character. Of course you should have strong character too, so even if they want to cross lines you don't.
7) Listen to your spouse.
If my wife says, "No!" to the friendship, then it's a no. Period. If you don't like that, then I would advise you to never get married. It's not for you. Part of what makes marriage work is submitting one to another.
A spouse is not only a companion, but also your protector (in a healthy relationship) and they help keep you safe and protected from others and even from yourself.
I unfortunately missed out on many opportunities for valuable rich freindships with women because of my religious fear-based upbringing. "Don't even look at another woman, and definitely don't talk to them, and for damn sure don't ever be around one without your spouse present!" Oddly enough many of those same people who spouted that fear-based teaching ended up having affairs, being addicted to porn, and living a lie. Living that way obviously doesn't keep people from having affairs.
The reality is anyone can have an affair, the question is would that be beneficial to those closest to you? Is it what God wants, I mean really? You know the answer. No! No!
Please post your thoughts, questions, disagreements and opinions!