YOU don't have to associate underwear with shagging.
But they are pieces of clothing that cover the shagging parts of the body. They are sexual parts in many ways (but not exclusively) and as are the parts involved in sex, there is a massive lingerie and sexy male pants industry.
I'd be very surprised if you weren't aware of it and suspect you are being disingenuous.
You've not seen lingerie anywhere? Ann Summers? Or Victoria Secrets catwalks? Or others like Fenty? .
Or just seen underwear in Debenhams or M and S or primark that made you think a lot of people did see underwear as part of human sexuality which includes shagging?
Intimate parts of the body.
And the Intimate parts which aren't just for shagging but where the most private bodily functions happen.
Shitting, pissing, menstruation, vaginal discharge and spotting at different points in the menstruation cycle.
They're Intimate pieces of clothing, not just 'items of clothing'. If they weren't, we'd all strut about in them in front of anyone and everyone.
But we don't. In fact, most people have a very small number of scenarios where we do let other people see us in underwear.
And if other people saw us in our underwear without our consent e.g men peeping in changing rooms then they would be committing sexual offences.
As would men nicking knickers off washing lines..
They're not 'just clothes' or 'just laundry' they have lots of implicit thoughts, feelings and societal and cultural attachments.
If OP had posted 'I found a pair of my or DDS knickers in DS room and I do the laundry and definitely didn't put them there' MNetters would be falling over themselves to say he was already a pervert or at the very least in the making to be one and OP should report him to anyone that would listen
But when he's expressed mild discomfort about handling his Mum and sisters knickers, suddenly knickers mean nothing Intimate to some posters and he's just an asshole gearing up to be a shit adult man who will be worth nothing to nobody 🙄