To the degree we leave our first love (God) is the degree we leave our 2nd love (spouse) .
You've hit the nail on the head here cross...
There are a whole host of issues surrounding this, but the main thing amongst a Christian couples is the lie of individualism in western society.. its all about ME. MY happiness, MY joy, MY time, MY freedom..
You also have the positions where both where not christians when married, one becomes saved the other does not.. The unbelieving partner may well want to leave. That's not the fault of the Christian... but you have to ask how often is that used as an excuse?
It's not an easy issue to answer, However, our prayer life should involve praying for those in the church (your own local church) who are married..to stay married.. we should be shining examples of light and love.
Family is community, community is society..as marriage in the real sense crumbles so has society been.
I think on the whole christen marriages collapse because of the ME ME ME syndrome...