But is anyone saying that "they tell their partner everything and feel guilty even speaking to another man"? I don't think that's what posters are saying at all.
As far as I can see, people urging caution in this situation are making a huge distinction between the life enhancing male friendships they have with men who are also friends of their marriages - and those that actually want to harm their marriages.
Fortyplus, I have been in the situation that you describe several times. Male friends have asked to see me - on my own - to offload about something that's troubling them. Like you, I've told my DH that's where I'm going, but if the said friend has asked me to keep a matter confidential, that's what I have done. It hasn't troubled DH - and it hasn't troubled me, because these individuals are no threat to my marriage. And the key to all these interactions is that they are not held in secrecy.
Then there's that grey area in the middle - as there always is. I've been married 25 years too - and in that time, I know I've had friendships with men who with hindsight, clearly weren't friends of my marriage, because at some point it became clear that they actually wanted me to have an affair. My approach to those situations has been to tell DH what has been said - and he has trusted me to deal with the situation, by making it clear that I'm not interested. Those situations haven't troubled me, because
I haven't been in the least bit tempted to take them up on their offers.
There have been other friendships when my internalised reaction has troubled me - I know I'd have been tempted if an offer was made. That's been the line for me that I won't cross - so I've backed away and not put myself in a situation where my resolve was likely to weaken. There was a time when I wouldn't have told my DH about that conflict - now I would, because as a couple, we have come to the conclusion that we are both going to find others attractive and it's human to be tempted. Talking about those situations openly removes the secrecy - and our relationship is secure enough to have that adult conversation.
There is a world of difference between those situations and friendships with men that are conducted so secretly that our partners wouldn't even know we had the friendship - never mind the secrecy about the interactions therein. The point you are perhaps missing is that there is generally a reason for such secrecy. Despite the OP's protestations (and sorry, I still think this is memorylane communicating with her exH via a secret SIM card) that there is nothing to this other than friendship, there is a reason she is keeping this secret.
There's nothing 1950s about this at all - in fact it is very much a 21st century problem with grey areas that to an extent didn't exist in the 1950s, with the advent of social networking sites and collegiate relationships in the workplace.
What hasn't changed one jot is the thrill adults get from something that is secret and illicit. Why do you think the poster called this an affair? It's because deep down, she thinks it is - or has the potential to be one. No-one in their right mind would ask the internet if it was a problem having a male friend - but this poster has at least been honest that she wouldn't like her husband to be conducting a similar secret friendship. It's a pretty good rule of thumb in any relationship, as AF says, that you treat people how you would like to be treated.
Finally, Pagwatch's milkman analogy made me laugh out loud yesterday thank you Pagwatch!