If best friends become lovers the best friend is lost if the relationship/loving part is over.

Dilemmatic. Ain't it? 2 things lost at same time: best friend and lover.

Also men seem to use everything they know about their ex in dirty ways in court ...

It is sad. But many examples to say is true.

... so it's better to not marry the best friend and keep the distance.

Or maybe to keep the alert about whoever you are married to.

"Please don't leave me" is not as powerful as "please leave me".