We've historically treated the bedsheets of the ill as being an infection vector, before germ theory. It is odd they tie bedsheets to sex contact alone. Eg if

We've historically treated the bedsheets of the ill as being an infection vector, before germ theory. It is odd they tie bedsheets to sex contact alone. Eg if you sometimes co-sleep, that itself is a vector. Though good chances are you are also having sex with your sleeping partner. There is a time element as well. If someone with the disease laid out on your sofa yesterday and it's dried out since, you're fine. It also probably requires sustained (and probably wet - eg sweat) contact.