Hello Richard

You're certainly not the worst person ever.
When I was a teenager a very similar thing happened to me.
Do you have OCD by any chance?
Truth is, semen is one of the cleanest and most harmless substances on the planet.It has to be, in order to give the genetic material inside the greatest chance of 'survival'. Semen is not 'dirty' or in any way harmful. I think semen has that 'eww' factor nowadays purely because it is related to sex, which isn't the most comfortable subject. Personally,I'd be more worried if you got ketchup or most other substances on those CDs.
There is absolutely no reason to feel guilty and, if you don't have OCD, forget about this and enjoy your day. You've done nothing wrong. If you do have OCD, I suggest looking into some 'mindfulness' meditation or, if your condition is very debilitating, then look into 'CBT' therapy. But this should be a last resort.
All the best and I do hope you feel better.