Explaining marriage to my soon to be step child

OK guys I'm just looking for advice. The other night we were talking to my fiancé's son about how we were going to get married and what marriage meant to us. we found the person that we love unconditionally and that we wanted to be together for the rest of our lives. My soon to be six-year-old stepson said "but my mom and dad were married and they didn't stay together forever."This really stumped the both of us and my fiancé started to explain that they didn't get along anymore and things like that. I've been thinking about it the last few days and just don't really know how to explain that this is different and the importance of marriage and how I want him to feel about finding a future soulmate in his life.