Tissues and Sex

Why is it that tissues are always found after sex? For example, in Ebichu. Is it because they used it to clean up?

the other day i found one of my old cumshirts, hell of crunchy and nasty under the bed, i'd say it's about three months old.

so, what should i do, keep it as a memento of the last time i had sex, wash it and wear it, wash it and delegate it to perma-cumshirt status, not wash it and wear it (stains are on the inside, could probably pull it off)

who knows, not washing it might set off some pheromone shit to make me irresistably sexual to all species and genders