My wife and I met over 40 years ago largely because of cycling and we remain active cyclists. We've toured up to five months at a time together and it would be hard to find a better team. Cycling has pretty much defined our lives together. A cycling lifestyle through our working years led directly to early retirement and enjoying a free and simple active life.
We've met other couples on long tours, some are closer than us, some have split up. A long trip can be a crucible, which can be strengthening or destructive. It's certainly a good way to find out what a person is made of. The same is true of platonic relationships, by the way. My hiking partner of twenty years and I are inseparable buddies.