If you weren't raised Japanese, with fundoshi being a fairly unremarkable part of everyday life just like tabi socks, kimonos, or obi -- or, occasionally, the de rigueur dress code as in the case of Hadaka Matsuri ("naked festivals"), no doubt fundoshi are more than just a novel cultural approach to "girding one's loins" -- they are a bit of a puzzle; questions like "how do you wear a fundoshi" often travel hand-in-glove with "why would you wear a fundoshi?" Can it be comfortable? How do you go to the bathroom? How do you wash them? And what happens if someone catches you in your fundoshi -- how do you explain?
The answers are fairly simple and straighforward:
As for "why would you wear fundoshi," I can say from experience that the support and comfort are easily acclimated to -- and well worth the period of trial-and-error that every neophyte fundoshi enthusiast goes through learning the ins and outs of a well-wrapped fundoshi. Fundoshi are simply the most ingenious solution ever devised for concealing and protecting the male apparatus, blending the advantages and appearances of briefs, thongs, jocks, and loincloths all together into one supremely sensual and utilitarian garment. Fundoshi's association with the martial arts, Shinto, and military and warrior classes heightens their masculine appeal, investing the wearer not only with sexy mystique but also with a cultural history that prides itself on inventive approaches and elegant solutions -- even down to their traditional underwear.
With a little practice you'll find the perfect snugness -- the tightness that will not only flatter your body, but keep you contained and hold its shape over the course of a day. Too tight and it won't feel comfortable -- if you feel like the fundoshi is mashing your balls into your body, that's too tight (unless you're into that, I suppose). If things are falling out or if it doesn't feel secure, that's too loose (although a fundoshi tied on with slightly relaxed tension is superb for lounging or as sleepwear, where supportiveness isn't a concern.
One major difference between fundoshi and underwear is that fundoshi aren't elastic -- there's no stretch whatsoever. I've experimented a little with stretchy fabrics for fundoshi, with mixed results. Lightweight cotton jersey is soft and comfortable, but as it stretches it has a tendency to stretch out. So there's some periodic snugging and adjustment needed to keep it comfortable. A stretchier, more elastic fabric with more "memory" could be interesting to wear, although since it will always want to contract it might actually tighten on the wearer after the initial tying.
Traditional rokushaku fundoshi, to be sure, are non-stretchy, made of lightweight cotton, linen, or (in some cases) crepe silk. The design of fundoshi uses this quality of the cloth advantageously, forming a secure, non-slipping wound "belt" that passes around the waist or lower belly, and the "pouch," which contains and supports the genitals.
Being a thousand-plus year-old design, no doubt the question of eliminating waste has come up before for fundoshi wearers. It's really not that tough -- don't think about it too hard! In the case of urinating, the pouch can simply be slid to one side, the penis extracted, and everything tucked back into place when you're done and cleaned up.
#2 is perhaps more challenging, in part because of the previously discussed non-elastic fabric. Since the fundoshi is basically a woven knot of wide cotton ribbon passed around and between the legs, tightened, and wrapped around itself firmly -- getting out of your fundoshi is necessary for defecating. So don't wait till the last minute!
Also (and although this seems obvious), since fundoshi is worn close to the skin, in fact actually passing over and against the outside of the wearer's rectum, one should clean up thoroughly before re-tying the fundoshi. In fact, if I have the luxury, I like to use a bidet or take a quick shower afterward. If this is not possible, just be scrupulous and attentive and you'll avoid skid marks.
Another byproduct of the no-stretch aspect of fundoshi is that, like anything worn close to the skin, the fabric can potentially chafe. This is especially a concern when it's hot or humid, or if you're walking, moving around a lot, playing sports, or excercising.
One simple solution is to take the "apron" portion of the fundoshi (which is normally wound around the "thong" portion of the fundoshi and tucked under one side of the "belt") and, instead of winding it, simply bring it backwards between your legs and tuck it into the waistband flat, adjusting it till it's snug (see picture below).
This does two things: first, it puts a layer of cloth between your skin and the knot at the small of your back; and second, as you pull the excess apron flat against your buttocks, between your skin and the thong portion, it will separate the thong slightly from resting directly against your anus. This can be more comfortable if you are being moderately active, however the traditional winding method is still recommended if you are swimming, doing something strenuous, or practicing martial arts.
Washing fundoshi is not materially different than caring for or laundering any other undergarment. Since fundoshi are worn next to the skin, a fresh fundoshi should be worn every day.
Laundering your fundoshi offers an added bonus: if you handwash or machine wash fundoshi, then tumble-dry them, they will soften and grow more comfortable. Don't worry about fraying along the edges -- any extra strings or fibers can simply be pulled off -- moderate fraying just makes your fundoshi more comfortable. Additionally, vigorous washing and drying will thin the fabric over time -- which is actually desirable as it will increase the softness and breathability of your fundoshi.
Also, a warm fundoshi fresh out of the drier feels wonderful, and is a great way to prepare for bed, especially if the weather is cold.
You may notice that if you have two or more fundoshi in the drier for the same cycle, they will braid themselves together. This is an interesting phenomenon which I haven't quite figured out how to avoid -- so a little untangling can be necessary while you're putting laundry away. It's interesting how uniform the braids of fundoshi can be -- almost as if they have been braided on purpose!
One possible solution that occurred to me is placing your fundoshi in zippered mesh bags that will keep them from tangling up with eachother.
Finally, as for what to do if someone surprises you in your fundoshi, well, that one's ultimately up to you -- you could turn beet red, you could just act normal and play it off as if nothing happened, explain that it's traditional Japanese underwear and actually quite comfortable (not to mention considered "full dress" and entirely modest all by itself), break out some sweet dance moves, or simply do what this fellow does:
(FYI, it reads right-to-left)