I just tried looking up the percentage of childless marriages. I got a lot of inconsistent figures, but nothing higher than a figure of 20%, which would include all voluntarily or involuntarily childless.
Compare that with the percentages of marriages with instances of infidelity, and it appears inescapable that a majority of those who have extramarital affairs have children at home. I suspect that parenthood does in fact reduce the likelihood of infidelity and promiscuity within marriage, but if it "puts the brakes on", those are some very faulty brakes.