Neo. The Only. The One.
THE ONE
It depends on the level of commitment. Obviously I wouldn't have an open-relationship with my wife, but if not married I'd be able to take my ego out of the equation enough for it to work.
But yeah if it's a woman I cared about enough to marry, then this wouldn't be on the table.
I'm a selfish lover. I don't care who she is. The second I find out I'm sharing her, she's gone.