I agree with this. No married man should be heavily involved emotionally with any woman who he isn't related to/married to. It's rude and disrespectful towards his wife, and very thoughtless. I'm pretty sure that most married men would not tolerate their wife having a cosy little 'friendship' with another man, going on 'early morning dog walks' with him and being a shoulder to cry on for him, and spending time together - just the two of them, socialising together and being alone together. (And constantly messaging each other - because this always happens when a married man has a female 'friend.' 🙄)
I am not saying a man and a woman cannot be just friends, but when everyone else is excluded, and the female friend of the husband doesn't know the wife (and in many cases they don't want to,) it's unacceptable. If they do know each other and the female friend is quite friendly with the wife, then yeah, her and the wife's husband being friends/socialising occasionally is fine.
But even then, it's unacceptable, and frankly weird, for a married man, and a female friend to spend lots of time together, texting and whatapping each other all the time, and having lots of private time together. I don't care what anyone says, its just weird, and wrong and unacceptable.
I have 5 female friends who I am quite close to, and I don't do this with them! We just meet for coffee or a pub lunch every month or so, and meet at social groups. We don't constantly message each other, and spend hours together having long private chats, and intimate 1-1 meet ups.
So why would a male friend and a female friend do this? (Especially if one - or both of them - are married/in a long term relationship.) Funny how 2 female friends, or 2 male friends don't spend lots and lots of time together (alone) sending lots of messages and investing lots of time in each other, but a married man and his female friend do. 
My DH has had a few coffees in costa with his lovely 33 y.o. female boss on their lunch break and she is one of the loveliest people I have met, and a really good manager. She is always very sweet to me. I have no issues at all with the two of them being together for coffee now and again. They are colleagues - and friends. The occasional coffee, and the occasional work related whatsapp message is the extent of their contact outside work.
What DID piss me off (around 6-7 years ago,) was this one woman DH worked with who was around 45, divorced twice, and had just been dumped by a man she had been with for 5 years. She was crying on my DH's shoulder, texting and whatsapping him, sometimes for 2 hours every evening! and keeping him at work after his shift had finished to chat to him about her problems.
She also started turning up at 10pm at work when she was not on shift and he was working nights, and sitting in the office with him pouring out all her sob stories and problems on him, and not leaving til 1.00am. DH had no romantic interest in her (he says this and I believe him,) but she definitely wanted more from him. Maybe just an emotional affair, maybe a shag, but the 'friendship' with her made me very uneasy.
I ended up telling DH, (after 2 to 3 months of tolerating it!) that I was finding it all really disrespectful towards me, and why is he investing so much time in her? It's like he was more concerned about her than me, and he was really invested in her problems. I think he saw himself as some kind of 'knight in shining armour.'
He was mortified and said he was just 'being a friend.' Anyway tl;dr, he told her he's taking a few steps back as he can't keep getting involved with her personal life and she needs to stop coming at night. Long story short, I got a really nasty spiteful message off her on facebook, telling me I was laughably jealous and threatened by her, and that I was keeping DH on a leash, and he will break it one day and I'll be sorry. Funny how I had no issues with his (younger) female manager who he had lunchtime coffees with. 🙄
I didn't respond I just blocked her. I showed DH the message and he had a go at her and started ignoring her work and said he no longer wants to talk to her. She left not long after.