Poor genetics. They must be so tired of everyone blaming them for all the negative aspects for their lives: obesity, tendencies towards addiction, and now sluttiness. Well, okay, not sluttiness, but a propensity for sleeping around, one-night stands, and infidelity, which pretty much sounds worse.
According to scientists, the dopamine receptor gene DRD4, the same gene that predisposes people for risk-taking, can be a significant factor in whether or not people sleep around or cheat. Although they say you can not completely blame the gene, it may be a factor into whether or not people will hold to their vows.