Seriously, you never heard of this type of behavior? I did and I was in school before you.
In my years in college ('83-'87), we saw a significant change in behavior because of HIV. There was a huge hook up culture my freshman year (not just among freshman). By the time I graduated, people were afraid. The reality of death due to casual sex did have an impact.
But STDs have once again receded from the radar, and of course young adults think they're bullet proof.
And a media that encourages casual sex and cohabiting before married doesn't help.