I, in contrast, tend to think everyone is clean, sober, law-abiding and monogamous.
A friend of mine just shakes her head and smiles at my obliviousness! 
Assume people are, at the core, not only selfish, but also too stupid to get out of their own way, and you'll be right more often than not.
My friend tells me, "The reality is, most people are drinking and drugging and fucking
anyone they can get their hands on," and I just look at her in bewilderment. 
Not *totally* true. Depends on the demographic of people. Though in that triad of behaviors (this is true across demographics) if you have the first two you prolly also have the inadvisable sexual behavior.
