When they do they over do it and put themselves in vulnerable situations that they regret later.
Everyone knows about the alcohol-laced 2 AM booty call, or the drunken stumble out of a club with the guy you just met.
(Perhaps that's less true for US college students today than when I was in college, because the drinking age is 21 rather than 18, but I assume drinking ages still aren't enforced very strictly except at bars, nor are marijuana-smoking ages.) Some of them may be drinking to get the courage to talk to other people, or to get drunk, but even those who aren't probably had a drink.
The bad part is sex that wasn't consensual because one or both partners were drunk.
There was an almost perfect linear relationship between drinking and partner closeness: The less known the partner, the more likely women drank before sex, and the more likely they drank a lot. When the casual partner was an ex-boyfriend, for example, only 30% of hookups involved drinking and 17% heavy drinking.
When the partner was a random stranger, however, 89% of hookups involved drinking and 63% involved four or more drinks!