Ppl continue to act like men and women can be friends but that primary issue is everything that makes a good friendship also makes a good relationship
All relationships naturally progress until they can't and the natural progression in these setups is for it to become romantic

The only difference between a close platonic relationship and a romantic one is the sexual/intimacy factor
If there's nothing to prevent that feeling from developing it will ALWAYS go there

Men and women can be friends but there has to be something that blocks the progression of the relationship
Where people fuck this up is assuming that block occurs between the two people in question, it doesn't it has to be external kmoy

A guy can be friends with his best friends girl because his relationship with his best friend is the block
His relationship with his best friend and the respect the have between each other fundamentally changes the relationship between him and his best friend's gf

This is why it's important for people in relationships to be mindful of that opposite sex
These outsiders want to be friendly but the idea of your spouse doesn't mean anything to them unless they know them
"I'm married" doesn't mean shit to A LOT of people