Guidelines
Dating within the League makes sense. After what happened to Ralph's wife, and then what happened to Clark's, there's a feeling that bringing non-Leaguers into the kind of dangers associated with their lifestyle is unfair. There is no official rule, simply tacit approval when Flash breaks up with his human girlfriend and starts dating Kara. They aren't alone.
Some marriages occur. So do some divorces. Schedules change to accommodate old wounds and new liaisons. They forget, or ignore, the new dangers. Dinah is the first widow, Rex the first orphan.
Ten years after Bruce left her, Diana finally understands why.