Emad -
Hosts charge a cleaning fee to cover laundry costs etc. If you think guests are not going to leave stains on sheets whether couples having a romantic time or a lonely single guy, you're in for a bumpy ride.
As hosts, just like hotels, we deal with laundry issues all the time. Things that I have to accept but annoy me are female guests who use white towels to remove their make-up despite face cloths being provided.
And once I experienced a really horrible situation when drunk adult guests wet the bed, said nothing and I had to replace entire mattress and bedding. They did pay for it - it wasn't a malicious action on their part - it was an error of judgement brought about by high alcohol consumption.
So as you can just dry clean the bed cover to remove the problem, you should do that and learn from it. Make sure you have waterproof mattress protectors and charge a cleaning fee.
As a host you can't get so sensitive and offended by bodily fluids (unless it was faceas, and clearly deliberate) so best get used to such things. And if you placed a review for this guest accusing them of being disgusting for leaving a stain on the bedding, chances are other guests would be too nervous to stay at yours particularly if they are a couple.
You can also leave private feedback and if you feel minded to, you can ask the guest to pay for the dry cleaning of the bed cover that was stained. they might or they might not. Personally, I wouldn't bother.