Some people like to keep their sheets and mattresses as free of fluids as possible. If that’s true for you, try laying a towel on the bed before you have sex. Some people do this as a way to avoid getting various sexual fluids on their sheets either because they don’t like the wet spot or they worry about trapping fluids in their mattress.
I am not aware of any scientific research that has been published in peer-reviewed journals that suggest any of these methods actually work to make semen or vaginal fluids taste sweeter or generally more pleasant. That said, I am also not aware of any research that discounts these methods – in other words, they are pretty much untested and unknown.