The thin cross line is when any right has being restricted or abused. For better and for worse is used to remind couples that everyday they'd spend together won't always be rosy,happy,some of the days will come with misunderstanding, quarrels, etc.
It is used to tell couples to still love each other when things aren't all tha good and romantic anymore. To remind them that they are both in it together.
This is true. But I guess some people got the wrong meaning.