I have some rather strong thoughts on this.

Male friends I think are okay, but as soon as they evolve into being a confidant they are problematic. If you have to turn to such a male friend at a time when you are in emotional need instead of your partner you have crossed a boundary and I believe the relationship has become inappropriate. If you have to hide any aspect of that relationship then it is inappropriate. If you have to start being secretive then that relationship has become inappropriate.

The litmus test is: how would my partner feel about this? What is their expectation?

If at all your answer is that he probably wouldn't like it then you know what direction you must go.

Friends of your partner's same gender are perfectly okay - but I think one needs to be completely open about them with one's partner. Not talking about them in front of your parter sets up alarm bells. Conversely talking about them and conversations you've had might make the relationship less suspicious. For example if your partner knew you were in contact socially with a guy at work and you never mentioned anything about it he would probably be uncomfortable. But saying yeah I spoke with such and such yesterday and he was telling me he had a bad day, would be a lot less threatening.

I am rambling. I hope this makes some sense.