I think when all is said and done, it's not about the underwear, it's about him not listening to what his wife is saying/asking him NOT to do.
It's wilful ignorance. Do a task, so it badly, "oh I didn't know own sorry" is easily forgiven if someone isn't doing it over and over.
So you add the ruined knickers to him say, moving pieces of paperwork that you need to "tidy" them, but then not remembering where he's put them. And then eating the last two eggs for his dinner when you've said to him that morning "don't use those eggs, I'm going to need them for a meal for everyone this evening". And then putting things away, but not in the place where they go, so you then have to look for them. Or giving the kids a biscuit before they have their dinner, so then the don't want what you've cooked.
Each instance you can go "oh FFS!" And dismiss it as one of those things. Bit when it's every day, it gets wearing.
You needed the paperwork to make a phone call, you put it there specifically and said "leave that there because..." now you can't find it, so you can't complete the task. So you then have to locate the paperwork (again) taking time away from the actual doing of the task, he can't remember, in fact he'll swear he's not had them, so your having to second guess all the time. He gets offended that you're accusing him of moving something he "hasn't had."
You don't find out he's eaten the eggs until you're already cooking and you go to lay hands on them. So you either have to work around not having any eggs/stop and go and buy eggs/have a row about him having eaten the eggs and send him.
You laid out a load of clothes and items to go to the charity shop. He "helpfully" goes and puts it all back. "I've tidied that pile of stuff away for you" even though you said "I'm going to drop a bag up to the charity shop later, put anything you want taking away in that bag"
You've just made a meal, the kids were moaning they were hungry, you nip up to the toilet and by the time you get back, they're eating a KitKat each. He's given them one "what did you do that for?!" "oh they were hungry" "But I've just made tea!!" Wasted time, effort, money, all because he didn't read the situation.
When you have to constantly allow for someone else's fuckwittery when they don't listen to you, no matter what you say or do. Or when you repeat yourself to make sure they've heard/listened "stop nagging". You can't win. When it's every day, for years, it's exhausting.
Can you tell my DH does the same?! He would tell you he adores me and tries his best. He doesn't, because he doesn't listen. Just yesterday, "don't give the baby another banana" simple enough right? He gave the baby another banana, because he wanted one. Of course he wants another one, he's a baby. But actually too much banana gives him stomach ache and then the shits, and it's me who will sit up with him all night while he's writhing in pain and the inevitable nappies. I've TOLD him all this before. He just saw it as "the baby wants a banana" and none of the consequences because "it's only a banana". But it's not, it's the baby crying, no sleep for me, shit everywhere and a bad day of an exhusted, grumpy one year old the next.
It's not about the knickers, OP could by knickers spun by Andalusian mountain spiders from the gossamer hand picked by fairies and kissed by a unicorn. Or bog standard five for £3 from Asda. Its not about the knickers.
If she had asked "would I be unreasonable to leave my dh, who has no respect for my belongings and doesn't listen to me no matter what I do, despite me taking measures to make sure my belongings don't get ruined by him? I'm sure she would have got a different response that outrage about hand washed knickers.
If he smashed her car everytime he drove it, or put her iPhone through the machine on a regular basis, or a whole host of any other things, she would have had a different response.
She shouldn't have to hide her stuff in her own home, or not have nice things because a sentient, adult human can't follow simple instructions. Simple instructions that have no impact on his life by not doing them. Simple instructions that have a negative impact on her life by him not following them.