Tough question and tough call. As with many things, I believe you need to look on a case by case basis. There are many factors to consider including why a person married another person.
For example, if you were looking for someone who was "real spiritual" per the Watchtower Society's viewpoint, and that was a high priority or number one quality in a person you were looking for spending you life with, then sure leaving the organization has a high probability of the marriage breaking up; especially if there were no other desirable factors besides the Watchtower Society's definition of spirituality.
If you were marrying strictly for the sex and physical attraction, then sure, more than likely that marriage will fail whether you're a Jehovah's Witness or not.
However, even with the above considerations, depending on what both parties bring to the marriage while in it and how hard both are willing to work on the marriage is also a huge factor.
Ultimately, though in the case of a Jehovah's Witness, if one believes the organization's teachings whole heartedly while another does not or no longer believes the likelihood of breakup, divorce, or a miserable life remaining together is extremely strong.
I was fortunate enough to know my wife as a person prior to her becoming a Jehovah's Witness, when she was a Jehovah's Witness, and even now after we are both not Jehovah's Witnesses. When I told her I loved her unconditionally, I meant it as did she. For both of us, we were both blessed.
Sadly and understandably when some Witnesses tell each other they love them unconditionally, they are lying to one another. The condition is as long as you remain a Jehovah's Witness. That is true whether it is a husband and wife relationship, parent and child, sister and brother, etc.
Again though, there are no set rules.
