There's nothing illegal about wearing an adult diaper as a man or a woman or at any age.
People of all ages wear diapers because they need to wear diapers to manage incontinence and as long as you wear the diaper under your clothing there should be nothing illegal about it.
You don't have to completely hide the diaper but just don't go out in public wearing nothing but a diaper just for common courtesy.
Although it's not really illegal in most places to go outside in public wearing nothing but a diaper it really is not a nice thing to see either.
Yes toddlers and babies do go outside in public in just a diaper but even then it's not really nice and I would prefer that even a baby or toddler have shorts on or a onesie on at least to cover the diaper.
I have seen a few adults out in public wearing nothing but an adult diaper and cops have walked right on by them without saying anything and I never said anything myself but it's best to wear clothes over the diaper.