It might be that relationships can last successfully for 50-60 years but not for thousands of years -- long-lived people could have many relationships, each as long as our longest marriages.

Having several hundred 50-year relationships actually might be interesting. You have enough time to get to know your partner deeply and intimately, through fifty years' worth of life stages. It wouldn't be the "post-Singularity equivalent" of a one-night stand, because you actually do have fifty years to learn what makes that person tick, in all his subtlety and complexity. But you never have to worry about feeling trapped because hey, it's only fifty years, you've got lots more time.