Meh, I don't think so. I have had mistresses while loving the mains and thought nothing of it. The mains stop doing basic things in the bedroom department, so I had to venture outside to find it.
Eventually women start to ration off the sexscapades from when you first met. First they would take it all ways for all days(kitchen counters, bathtubs, etc.), then head only happens on special events, then missionary and no head, then "Can you hurry up already?", and the finale is no sex at all. Sorry ladies, if you can't keep a guy interested sexually, you are going to lose them.
Men are inherently conquerers and women play the long game and get emotionally invested when they creep.
I've crept with two jump-offs in the week and went home to the main and slept like a baby...it isn't cheating to me unless you get caught IMHO...