I was stunned to discover that the survey currently considered the gold standard in trans research, "Injustice at Every Turn," which was conducted by The National Center for Transgender Equality and the National Gay and Lesbian Task Force (and which surveyed 6,450 Americans who were transgender or non-gender conforming) actually does address the outcomes for marriages and relationships. And, shockingly, contrary to the accepted wisdom, only 45% of people reported that their relationship ended due to coming out or gender non-conformity. Among those who actually went on to transition, the rate is 55%. (Not all of the remainder stayed together, but in those cases the respondents reported that their relationships ended for other reasons unrelated to transition. I searched the original study report in case the "stayed long term" was broken out from "left for other reasons," but couldn't find it.) There are some more specific breakdowns by age, employment data, and children in the actual report :
http://www.thetaskforce.org/downloads/reports/reports/ntds_full.pdfAlthough 55% overall is not fantastic odds, it still flies in the face of the regular claims that relationships "never" survive or that it's incredibly rare. We're the majority.

And I wonder how much of this is a self-fulfilling prophecy - how many more partners would stick around if they weren't surrounded by cis people telling them to flee and trans people insisting that it's hopeless and they should give up. Those of us who stay are often stubborn and determined to fly in the face of conventional wisdom; the tragic part is that that conventional wisdom is WRONG.
Of course, as I've been saying all along, odds are not destiny, either - if you're both still in love and want to make it work, there's always a chance.