Fair enough. I see that as being no different than lying on the couch wearing sweaty, wet, dirty clothes after working in the yard on a hot day. Yuck. Clean up or put something down between you and the furniture, please.
Let's face it. Even clean, our skin is hard on furniture. Even if it's just bare arms sticking out of shirts and bare legs sticking down from shorts. But the bottom thing is another matter. You can fix that just wearing underwear and nothing else, but who wants to go just that far and no further?
Hence the towel thing. Or sheet, or whatever you can put down and clean separately. I contend that most towels are thicker than most of the clothes we often wear are. OK, it's a pain to carry a towel wherever you go in the house or have towels or sheets put down everywhere you might sit. If you have towels put down everywhere and someone unexpectedly comes over, it's going to look strange and if they know why you put the towels down, they're not going to want to sit there.
One thing that doesn't get mentioned much is if you carry and put down a towel, are you putting the same side against you each time, or are you allowing the towel to sometimes flip and possibly transfer to where you're now sitting? The simple fix for that is to use a towel that has different prints or patterns on each side and making sure that you're putting the same side against you wherever you go.
What about outside or at the beach? Do you still need to sit on a towel? Generally yes, though places that get sun exposure are quickly sanitized by the UV from the sun. Others ask isn't it more hygienic to wear a bathing suit in a sauna or on locker room benches instead of sitting nude on top of a towel? I put it to them this way... Would you rather someone sit on a bench in a wet bathing suit, having stewing that suit with whatever issues they have for hours, leaving the bench wet behind them to be sat on by another person likewise in thin swimwear to sit on, to stew in the same mix they then pick up, possibly for hours? Unless both people are wearing waterproof plastic, transfer is easily going to happen. Even a wet towel and nothing else between the two people provides better protection than that. Using a dry towel is certainly better.
So there you have it. Any other concerns are red herrings. Besides, towels are nice things to have with you at all times as described below from one of my favorite books:
A towel, it says, is about the most massively useful thing an interstellar hitchhiker can have. Partly it has great practical value. You can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a miniraft down the slow heavy River Moth; wet it for use in hand-to-hand-combat; wrap it round your head to ward off noxious fumes or avoid the gaze of the Ravenous Bugblatter Beast of Traal (such a mind-bogglingly stupid animal, it assumes that if you can't see it, it can't see you — daft as a brush, but very very ravenous); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems to be clean enough.
More importantly, a towel has immense psychological value. For some reason, if a strag (strag: non-hitch hiker) discovers that a hitchhiker has his towel with him, he will automatically assume that he is also in possession of a toothbrush, face flannel, soap, tin of biscuits, flask, compass, map, ball of string, gnat spray, wet weather gear, space suit etc., etc. Furthermore, the strag will then happily lend the hitch hiker any of these or a dozen other items that the hitch hiker might accidentally have "lost." What the strag will think is that any man who can hitch the length and breadth of the galaxy, rough it, slum it, struggle against terrible odds, win through, and still knows where his towel is, is clearly a man to be reckoned with.
Hence a phrase that has passed into hitchhiking slang, as in "Hey, you sass that hoopy Ford Prefect? There's a frood who really knows where his towel is." (Sass: know, be aware of, meet, have sex with; hoopy: really together guy; frood: really amazingly together guy.)
— Douglas Adams, The Hitchhiker's Guide to the Galaxy