Besides the familiarity, I think affairs actually reinforce the value of monogamy. Cheaters maintain the value of monogamy because even those who are cheating believe they're being immoral. Even cheaters believe in the value of monogamy, just not for themselves. I've had more than one person say it wasn't fair I had two lovers when they had one or none, but they weren't willing to accept non-monogamy as a moral good, despite the transparency.