I'm in a lot of agreement with what Barly says. A man or woman of God, who is walking with God, will not commit adultery, and that is because their focus is on God. Yes, at times we might slip. But adultery is not a slip. It is a willful act of following up on lust. Everyone of us knows what lust is, and sometimes we allow our selves to dwell on it. However, if we are following God, He will not tolerate us dwelling on it. Not even for a second. The minute we start to lust, God is already working on our hearts to convict us. Long before the physical act of adultery has occurred, the person who commits adultery has turned his or her back on God.
If we want marriages that are "affair" proof, then we need to be praying (not just for ourselves, but for each other), staying in the Word, and listening to God.