I've just finished watching the last episode of the last season of Babylon 5, and watching Delenn and Sheridan's heartbreaking farewell made me think that successful, happy marriages are quite rare in science fiction.
Captain Kirk famously has a few one night stands, but everyone knows that his heart belongs to the Enterprise (or possibly Spock, if you read some of the K/S fan fiction).
Picard, likewise, has fantasies of a happy family life with lots of children, but he too is firmly wedded to his ship and his career.
Sisko is a widower, and Janeway is unattached (I never got round to watching Enterprise).
The only happy, stable marriage I can think of over the whole of the Star Trek regular characters is that of Miles O'Brien and Keiko.
Star Wars, too, is noticably devoid of any married couples, let alone happy ones (unless you count Luke's uncle and aunt, and look what happens to them!).
In Firefly, there's Wash and Zoe, and in Doctor Who Amy and Rory are devoted to one another, but most SF shows involving space ships are based around a regular cast of unattached people thrown together by circumstances - like Farscape and Blake's Seven.
I'm not very familiar with the rebooted Battlestar Galactica, but in the original series, when Captain Apollo had thoughts about wedding bells, his bride-to-be was quickly killed off.
So it was lovely to see Delenn and Sheridan making a success of their relationship over the course of the series, and it would be nice to see happy marriages in other SF universes.