They got married in UK a few years back when he was studying there, but have been settling down here ever since. But being married doesn't mean they want to be exclusive to each other, instead they feel the need to be in a open relationship. They believe that sex and love is two different matters; have sex with all the men you want as long as we love each other.
Marriage certificate is just a piece of paper used to settle legal matters when one dies. It has no restrictive power to define how any relationship works. Though straight might feel the pressure to keep up with the norms of the society, the dynamics are just different for gay couple. It is silly to want the same things as straight couples when we are queer to begin with. Maybe it's just a matter of semantics.