My statement is that if the semen turned out to be Rudi's it wouldn't have made any difference in the case. If it had been an unidentified man's semen it would have turned the case on its head.
Regardless of the truth or falseness of what the stories were reporting the fact is that in the Skype calls both Rudi and Giacomo, his friend, referred to reports that semen had been found. Yet, Rudi denied that he had left any behind. If he had left some behind, why not make up a story to cover the deposit? Why not say that he had been fooling around and got so excited he came a little?
I think all agree that Rudi's story was designed to cover himself in every way possible. He was offered the opportunity to make up a story about the semen yet he demurred, actually vehemently denied. Why?
OK, I see your point now. Guede and Giacomo did indeed discuss sperm the police had found during their Skype conversation:
RG:
And then, we saw each other at Meredith's house and we started talking...we started talking, and I, well, I tried, I mean I made a pass at her and she was willing [ci stava]. But in the end we didn't do anything because...she didn't have any condoms and neither did I.
GB:
Like...in the newspapers it says that...well, you must have done something with her if...
RG:
Yes but in fact, I'm telling you we did just ...oral stuff.
Nothing, no penetration, because I didn't have a condom, we didn't do anything. And so it's all crap according to me that they said they found my...sperm...male sperm. Then I, that... it's not mine because we didn't do anything.
http://themurderofmeredithkercher.com/Rudy_Guede's_Skype_Conversation
Which further raises the question as to why the police never tested the semen's DNA, especially after the police had heard Guede deny it was his semen?
At some point around that Skype call the evidence against Raffaele started falling apart (his shoes, no DNA or prints of his found in Meredith's bedroom), which eventually sent the police scrambling back to the cottage in December looking for more evidence to test (the bra-clasp, etc).
Since the police were scrambling to build a case against Raffaele, I find it difficult to believe that the police never tested that semen stain, which surely had Guede's DNA.
So why was Guede so adamant the semen wasn't his during that November Skype call?
Maybe, Guede is as dumb as a brick and he didn't realize that his semen could be traced to him using DNA?
Maybe, Guede hadn't thought his story thru and Giacomo surprised him by mentioning it, so on the spur of the moment Guede denied he left sperm at the scene. After all, as far as Guede knew he was talking to a friend and didn't know the police were monitoring their Skype discussion, so Guede likely didn't feel anything he said to Giacomo that day couldn't later be changed to account for his sperm being there if he had to change his story later.
We also don't know what the Italian stories concerning that sperm actually said? They certainly had Guede's DNA to use for reference, and the media was already reporting they had found Guede's prints and other DNA in Meredith's bedroom by the time of that Skype call, so I'm assuming those Italian stories never mentioned that the semen stain had been tested and matched Guede's DNA, so perhaps Guede was going with the flow and he felt that the sperm wasn't traceable to him, for whatever reasons?
As it turned out, since the police claim they never tested the sperm, Guede never did need to change his story to account for his sperm being on Meredith's pillow, which is the larger mystery here.