Yesterday was my 32nd wedding anniversary. One comedian's quip often comes to me in this context. "50% of all marriages end in divorce. But you don't know. You could be one of the lucky ones. Your marriage could end in death."
Marriage is going to end badly. You may well fall out of love and continue to stay married, fall out of love and divorce, stay in love and have him or her walk away or stay in love and watch them die. Once you're in a marriage, there is no good way out.
But that is the worst reason not to get married. Whether it is marriage or romance, a business venture or Saturday adventure, worrying about how it will end is probably a great way to miss the whole point.