One of my best friends did this. His live-in GF was bisexual and brought home chicks all the time. Sometimes he got to join in. But he had multiple serious and casual relationships with other women as well, all with her full knowledge. The guy had a good time, all the time, as per Spinal Tap.
...until she went on a coke binge, drained his bank account, fled the country and made Interpol's 'wanted' list. I'm being completely serious.