I am gay so I have used condoms my whole life but my straight friends routinely go out and have unprotected sex all the time, I'd say about 5% of my straight friends consistently use condoms.
It's a design problem. Sex with condom sucks. Non-condom sex is 10x better.
The real solution will be when you have male birth control available as well; and then it comes down to only having sex with people you trust. I great partner is better than a bunch of lousy partners anyways.
I'm 29, and so far every woman I've fucked, I ended up deciding I didn't want to marry / have children with. But that's still on the table like it is for the vast majority of men, so vasectomy is not an option.
As someone who came of sexual age in the 90s, between HIV and getting someone pregnant I was (and still would be - I'm married now so...) terrified of sex without a condom.
The other big factor people don't like to use condom is that it greatly reduced the sensation, regardless how the condom companies have been advertising it.
That's horrible, and nonsensical for the time period. With the amount of advocacy, and information available. Why would ANYONE consider not using a condom in uncommitted relationships?
One reason is that the STDs you do tend to catch from casual uni sex are cured by a $20 pill, if that's even on your mind when you're drunk in someone's dorm bed.
"Shit, I got HIV/warts from hooking up" just isn't something I ever heard in uni. It just isn't part of the discourse. Those just don't seem to penetrate the uni bubble.