I think I can chalk this one up to my paranoid neurotic mother wanting me to get a glass when I was a kid, but her justification for my not drinking out of the carton was this: most milk enzymes are broken down by your saliva, and when you drink out of the carton, you’re getting some backwashed broken down milk back in the carton to ruin the rest. (Wow, that was a long sentence. Sorry.) Considering that I don’t immediately spit out half the amount swishing around in my mouth, I’m going to call bunk, but I figured I might as well ask.
I don’t know if the milk goes bad quicker, I’ve never tested it. But, if it does it’s more likely that drinking out of the carton inoculates the milk (via contact with your lips, and maybe some backwash) with some of the ~800 different species of microbe found in the average human mouth. They will probably love the conditions in the milk, especially if it’s kept out of the fridge for any length of time.
I remember being advised against feeding baby right out of the baby food jar for this very reason (some people feed baby a bit out of the jar then put the jar back in the fridge for later). Once saliva is introduced into the ‘common dish’, breakdown of the food begins. Anecdotally this seems to hold true for baby food; it will ‘separate’ and become watery if you do this.
This is a common urban legend, or, since it came from your mother, old wife’s tale. You also see it in the variant that the last swallows of a bottle of coke that’s being passed around is someone else’s saliva.
In fact, the amount of saliva backwash is so small as to be negligible. Saliva is just not very good in breaking down large quantities of milk in the first place. All that you would get is a glop of saliva in a carton of milk. Unappetizing, perhaps, but harmless beyond that.
One odd caveat that may have caused some of the initial confusion. Note this report from Snopes:
The problem there, however, is that breast milk is not pasteurized and it is the bacterial growth promoted by the saliva that is the contaminant, not a breakdown of the milk enzymes. I suppose if someone heard this and misunderstood the whole issue could be transferred to drinking cow’s milk from the carton.
But more likely it’s just another urban legend built around the need for sanitariness.