Actors are steeped in a world of agents and where the next job is coming from and what are their expenses and what is the hotel like. You want to take them out of that world and dump them into another world so that when you meet them on the screen they don't seem like the guy who was in two others movies that year.
Straight couples don't have to be monogamous to be married or married to be monogamous. Monogamy no more defines marriage than the presence of children does. Monogamy isn't compulsory and its absence doesn't invalidate a marriage.
