After giving this some thought, I do realise why some people, possibly more in the younger generation, can't see why many feel that this type of reaction between men and women is often deemed inappropriate.
Maybe there is a generational divide going on somewhat, and times are changing, but on the whole, (from the perspective of a 40+ heterosexual woman) a good proportion of men in this age group don't tend to search out female company to "chat".
Obviously every man is an individual, with his own likes and dislikes, but in my experience they tend to remain on topic. So while they are willing to talk about such and such a thing, whatever it may be, and will do so happily with men or women in a group context, be it at work, a hobby, sport or family and friends barbecue, to move onto messaging one to one about something other than a specific topic , sending gifs and comments throughout the day, is a different thing altogether.
In my opinion it's not about chatting and being friendly, or any particular topic, it's that on the whole, men don't check in with their male friends as regularly as women do. They may send information about something, they will discuss things, but they don't check in on their friends in the way that many females do. Again, I realise that there will be males and females who don't communicate in this way, but taken as a whole that's why I'd personally feel it was inappropriate.
I don't control who my husband talks to, obviously, but there's no way he'd ever spontaneously start texting with a female for no apparent reason, and if he did, in my world it would be a sure sign that we had problems.