Some good partners turn into utter wankers
This.
When your relationship is good and the man is good and considerate and helpful, many women simply cannot imagine that this man, with whom they are soulmates and best friends, will utterly do them over. I think most women don't realise that a lot of men's decency to them is dependent on them actually liking you and liking having you in their life. Once they no longer like you, or want you in their life, they are looking after themselves.
So when things are good, they mistake the man's decency to them as him being a decent man.
And lets face it, few of us are fair and good to people will do not like.
But all that seems unimaginable at the time when you are best mates in love.
And whilst this is a familiar pattern to those who hang out on Mumsnet boards, it not necessarily common knowledge to other people, who may never have known a single person this has happened to. I'm 50 and don't know anyone in real life this has happened to! So it just won't be a real and present danger to most women, especially when they are at the age of starting families, as everyone they know will also be at the start of their family life, not the age of disintegration of families.
Also, people still believe in common law marriage.