^ Isn't that the goal? I don't cheat, but that's mostly because I make sure that whoever I'm dating is attractive enough for me to not even consider it. If you settle and you KNOW you're settling, you'll always want better. It doesn't just apply to men, 'cause girls do it just as much: why commit if you have better options?
& I also find the second part of your statement interesting. While it might be unfair when they're aging, how about if she were younger? I see so many girls let go as soon as they find a man...then they wonder why he leaves. If you can't maintain the baseline physical attraction, the mental isn't always enough to hold it. And I'm not implying that it's all about looks, either - but after a certain point of getting too comfortable, people start looking (or rationalizing) for ways out - why give them that reason?