I need female/motherly advice only!!

A few months ago, I showed up to my 24 year old son's apartment. He wasn't expecting me. I opened the door to a big suitcase filled with bras, thongs, and panties. There were even pantyliners and maxi pads (new) there. They're different styles, sizes, and have all been worn. He told me he stole them from the apartment's laundry room. I bagged them up, took them home, and threw them out. I didn't count, but there had to be at least 100 pairs. I was shocked. And I embarrassed him because I didn't know how to react.

Not much else was said. I asked my sister, and she told me about "Panty Raids" and that he probably masturbated in them. He also hasn't had a girlfriend since High School, and that only lasted 1-2 months.

Howver, since then he's been living at home and I have caught him more than a dozen times, red handed, rifling through my underwear draw. He denied it every time, even though HE KNOWS I SAW HIM. I’ve also been noticing some of my kotex pads missing, too.
He takes from his younger sister's, 22 years old, too. We've both been noticing missing panties for well over 6 or 7 years now (she's a XS/S and I'm a M - for what it's worth, my son is very very slim), but never thought too much about it until now. My daughter over the last 2 years now takes ALL of her underwear back to graduate school with her, now I see why.
I guess the most relieving bit is that it always seems to be clean panties he takes. Thank god for that, I guess.

I think he's been doing this since he was a young teen!

I don't care that he has a panty fetish. What I do care is that he stole hundreds of pairs from an apartment laundry facility, and is going through mine and his sisters' undies.

How would you have reacted initially? How would you react if you saw your teen or adult son going through YOUR underwear? What do I do now?!

Thanks.