In this case I think the punishment he received is a clear deterrent... his bright future is totally gone, everyone who sees the case is not thinking, hell, I'll only get 6 months and be a sex offender forever, who cares let's do this! So the deterrent threshold is met in my opinion.
The real question is, is he irredeemable? And that's where you have to try to get a sense of who he is, and where being a Stanford swimmer comes into play in my opinion. He doesn't just happen to "swim real good", he had to work extremely hard at it (and academically as well). I think it means he's less likely to be a sociopath - you don't get to swim at Stanford if you're someone who is incapable of following the rules of society.
That's also where the drunken freshman factor comes into play for me. And this is all theorizing, but an elite swimmer with Olympic hopes probably had very strict parents - you don't become a good mid distance swimmer without extreme discipline, so there's a chance he never had a drink before college. Then you get to college and alcohol and women are everywhere.
I was a swimmer in college, I never had a drink before I got there. The first event the team had was in theory supposed to be a "workout", but when we got to the event, there was a keg and alcohol pressed on the freshmen until several threw up. The upperclassmen found it funny (and I did as well the next few years as an upperclassman). So you're a freshman, you're suddenly hammered and have no idea how to handle your liquor, and there's all these hookups happening. And men are taught to initiate these encounters and constantly push the envelope (this is just true; try being a man waiting for a woman to approach you and see how much success you have).
Again, the standard here is reasonable doubt, so I see it as a drunk freshman who's suddenly learning that some women might like having sex with him, but only if he initiates everything. He makes out with a girl who's blackout drunk, and takes things way too far.
I think the punishment is too severe, mostly because I don't see a drunk 18 year old thrust into that culture who did a terrible thing as totally irredeemable. It's also because, as others have said, the majority of the punishment happens outside the 6 month jail term, even ignoring the social media hate that's following him around too.
The registered sex offender and felon status he'll carry with him for the rest of his life means many of the opportunities formerly open to him are closed forever. There's nothing he can do to change his status. As an 18 year old it's illegal for him to go back to his high school and there are large areas where he is prevented from going. Most companies will never hire him due to the "Have you ever been convicted of a felony?" screening question. His life is ruined. But people want more.
It's interesting to me that liberals are quick to talk about how laws in this country are so extreme and how we have an incarceration/recidivism problem, but that sex crimes bring out such bloodlust. I'd hope we could be more forgiving generally. And yes, I'd have no problem with a longer jail sentence (say 1-2 years) if the sex offender status was something that could be forgiven over time for a first offender.