I blogged about that particular item awhile ago, pointing out its resemblance to the ancient Roman subligar which was worn by women, and an anonymous blogger commented that the Lengberg find had been identified as a man's garment. The blogger also claimed that a lot of 15th c German images show men wearing such garments.These pictures are not very frequent, and most of the examples I have found are from crucifiction scenes where the two other guys are shown as well, or from similar martyr scenes. One example from a crucifiction scene, from end-of-15th-century Austria, is here in the IMAREAL. If you search for "Kreuzigung" with the timespan 1300-1600, there is at least one more example. When I'm specifically searching for those undies, I am thus looking for crucifiction scenes - though those only showing Christ will not suffice, since he's invariably shown with a cloth covering his nethers, no modern underwear.
The University of Innsbruck's web article on the finds agrees with my anonymous blogger--it identifies the item as a man's garment, and remarks, I believe, that it was found close to a wool fabric scrap believed to have come from a codpiece. Given that the Lengberg finds did not come from graves, I don't think that sort of association, in and of itself, is persuasive evidence. But there may be other evidence I don't know anything about that supports the identification of the garment as male.
Can you point me to any images confirming that men wore such garments? The English and French images with which I am familiar show men wearing briefs that look more like modern Fruit of the Loom briefs, but I'm not very familiar with 15th German miniatures or artwork.
Sometimes the corresponding undies are also shown in bed scenes or in "battle for the trousers" scenes. Those topics are usually searchable in most piccie databases, so I'd recommend them as search terms.
(Technically, you can also search for clothing - underwear or underpants on IMAREAL. However, the tagging is not always complete or correct, and that search will not necessarily find you all the instances. This is not intended to bash the IMAREAL tagging - I have a private, tagged, picture database and I do know how hard it is to do it consistently and correctly all the time.)
Oh, and regarding the male-or-female discussion: We have proof (picture proof) that these types of underpant were worn by men, but that does not exclude them being worn by women. We just have no picture (or other) proof for it yet. So personally, I would say that it's not very likely that a lot of women wore them, but that they might have been used by some, or in some circumstances as for controlling blood during menstruation, for example.
I hope that helps!