The only way to fall "in love" with someone is to spend time with them.
This could even happen emotionally as in via correspondence such as the Internet, pen pals, or through a close "platonic friendship" between likeminded individuals.
It's important for one not to spend a whole lot of time with other women or men on a personal or intimate level. Sharing one's inner thoughts about the nature of their marriage with "secret friends" is often the gateway to having an affair. People become more emotionally invested in their friendship and less emotionally invested in their marriage.
However it's also important to note the difference between falling in love and falling in "lust". Just because one is married does not mean that they suddenly believe every other woman or man on the planet is unattractive or lacks sex appeal. If this were true there would be no such thing as movie idols. Lust is not love.
Admiring beauty in others never goes away.
Just because you have a new car doesn't mean you stop noticing other cars. The problem arises when you start making an effort to "test drive" them. That's when you start having thoughts of trading in your car. Imagination and obsessive thoughts lead to change.