I know that there are statistics "out there" that suggest that when one partner in a relationship gets MS (or other chronic diseases) that it can negatively affect the length of the relationship (expressed as a high divorce rate). Somebody recently even asked that question here...this is a paraphrase, but along the lines of "...now that I've been diagnosed with MS, how much longer will I be married?"
I'm convinced that having a dx of MS (or other chronic illness) certainly can put a strain on a relationship, and certainly can affect what goes on in the course of the marriage. For example, we specifically made a choice to not have children because we didn't know how MS might affect me, didn't know if I would have been able to care for children, didn't know how MS would impact our ability to earn a living and feed the little hungry mouths...so that was a decision that affected all our future together.
I also think that if the marriage is already "in trouble", there are already strains and "cracks" in the relationship, then adding the burden of a chronic illness in one partner can "break" the marriage.
But it doesn't seem to me to be an automatic link..."I've got MS; next step is a divorce." Heck, I've seen people divorce over even less serious things....and others stay together through lots of adverse events. We (my husband and I) have had job losses, long periods of time of unemployment of one (or once both) of us; a house burn down around us, a house flood out -- twice; serious injuries; deaths of parents; re-locating half-way across the country without having new jobs or a place to live to go to; my having MS and becoming severely disabled..
and yet...after 40 years this day...we're still together.