It won't effect your degree persay...but to be honest why should you care what other people think? If you wanna have sex have sex....just don't CATCH anything. That should be your only regret
Well, to be fair a lot of people won't be able to judge seeing as they'll have done exactly the same thing. And other students don't have much right to judge anyway, seeing as it's entirely your own decision.
It won't necessarily "ruin" your reputation, but I guess if it becomes public knowledge, more guys might try it on with you and other guys might be a bit wary of asking you into a relationship.
If it's a small Uni, everyone will know your business, and then you may get a rep - but then it isn't like you will be the only one having casual sex during freshers. If it is a large Uni I'd say go for it haha, you'll just be another student having fun before the studying begins.