Blame it on science
Although the tagline of the article suggests a 'free pass' for any guy naive enough to use it as justifications, Is infidelity ‘natural’ for men? is an interesting piece on evolutionary psychology, although it says really nothing new for those of us who are able to think logically. However, it's always handy to have a scientific study handy to debunk or explain away the stereotypes of 'gold-diggers' or 'cradle-robbers':

Many evolutionary psychologists say these divergent sexual strategies also explain two corollary findings of modern studies. One says men seem more disturbed by sexual infidelity and women seem more disturbed by emotional infidelity. The other says heterosexual men seek women who are young and beautiful because these are viewed as signs of fertility, while heterosexual women seek men who are rich because that helps in raising children.

I looked for the reason why it is impossible (for heterosexual men) to look at a pair of breasts and remain focused on... whatever you are doing (I lost my train of thought there for a second visualising)... but that will perhaps have to wait for the next study. There were more interesting results about the honesty, to self and others, that women have about sexuality:

In the study[she asked men and women to report whether they masturbated, watched soft-core pornography or hard-core pornography. Each “yes” got a point], both men and women had been told to hand their questionnaires to a researcher. But when women were told to deposit their answers in a locked box supervised by a researcher, their average score jumped to 1.53. And when the women took the test alone in a locked room and then deposited their answers in a locked box — ensuring privacy and anonymity — their score shot up further, to 2.04. The men’s answers did not change significantly, indicating they were less concerned about their opinions being discovered.

Ah, the old double-standard is still alive and well. It's fascinating to see it at work in the social setting, and self-reinforcing; I have observed that women are the ones who (for the most part) use words like "slut" and "whore" to describe other girls. It's a vicious and yet fun circle! Free your mind... slut!