Mixed-sex onsen

I read that you're expected to go into mixed-sex onsens fully naked if you're male while females can wear a wrap towel, is this true?

Mixed sex onsen are rare unless its for a private party or couple.

In onsen where women may enter with a towel, I don't think men are required NOT to wear a towel but men mostly choose not to. Women may also choose not to, but they don't.

If you are visiting a mixed-gender onsen, you can check the rules for that onsen. For example, Yubara Onsen has a famous open-air, mixed-gender onsen that now requires men to wear a towel over their lower half at all times:
http://www.yubara.com/wp-content/uploads/2014/05/img_Q5DzNY.jpg

Mixed sex onsen are getting rarer and rarer.

The small ones in the countryside in my experience everyone is naked. But yes, there are some bigger ones in Gunma prefecture where they now mandate kind of cloths (wraps) for both men and women.

I haven�ft gone, but as a women to sit in the water with a soaking wet towel doesn�ft sound like a lot of relaxation.

Most onsen nowadays are anyway gender segregated. (From about 6 years and up).

If you know the specific onsen you are looking at going maybe we can find out what the rules for that one are.

General answer: it depends on the bath! It ranges from one extreme (any cover-up prohibited) to the other extreme (bathing suits mandatory) and everything in between, including those that do not have clear rules.

You have to research the rules of individual baths. Do you have one in mind?

I've been to many onsen and all were no towel. I've only been to one mixed onsen which was also no towel. Guys on one side of the big bath and a lone woman on the other.
I thought the wearing of a towel was generally a misunderstanding. In the ad or on tv people wear a towel to avoid showing nudity but this is only for the ad or tv.
The only places I know with towels are more pool-like and for a younger crowd perhaps rather than for washing and relaxing.

some onsen where they are mixed onsen, have slight change in regulation, Takaragawa Onsen, although a mixed Onsen, day visit guest are require to wear bath dresses and towels provided by the establishment, at charge, you cannot bring your own.

Mixed sex onsen are rare unless its for a private party or couple.
If you are referring to kashikiri, they are not really rare. Additional fees normally required to use such facilities.
"����" Kon'yoku are not common; surprisingly, I have visited these places only to learn later that they have "����" Kon'yoku facilities.
https://www.takaragawa.com/  ;
https://www.kirishima-hotel.jp/  ;
http://www.hoshi-onsen.com/ ;
http://www.tsurunoyu.com/
https://www.oirase-keiryuu.jp/en/
https://www.namari-onsen.co.jp/
http://inbound.13gatsu.jp/  ;
http://www.yoroushi.jp/  ;
Kirishima Iwasaki Hotel  ( closed) 
Furusato onsen, Sakurajima ( closed)