Looking at your photograph, which may or may not be you of course, I have possibly been hosting since before you were born. So what you’re going to have to do is what every STR host, hotel, guesthouse, hostel and so on does.
That is, accept that people are going to leave evidence of their sexual activities on your sheets. Learn more about laundry - sex shouldn’t cause a stain. The word ‘stain’ means ‘permanent’ or ‘hard to remove’. Jism (a preferred word to @KKC’s ‘juice’ ( 
) doesn’t stain, blood doesn’t permanently stain, skid marks don’t stain.
However, are your guests doing something that is producing genuine stains not covered by the aforementioned three? If so, how is that? How are you attracting people that use purple dye (or whatever) during sex?