While the various reasons cited in this article can doubtless be found to apply here and there, it is probably worthwhile to remember that most of our behaviors are the result of hundreds of thousands of years of evolutionary hardwiring that on average was adaptive for our ancestors. Empirical studies have shown that women are more inclined to consider infidelity during ovulation, and that at this time men with more pronounced "male" characteristics are more highly rated as potential sex partners. This suggests that women who've secured a safe provider are then inclined to attempt to get more desirable genes (for height, strength, etc.) - even if, of course, they are entirely oblivious of such underlying motivation. It is not difficult to imagine exploring this evolutionary perspective more deeply, and thereby revealing a much wider range of hardwired feelings and behaviors.