I have been in the position twice in my life of being party to this sort of thing.
If it is what I think then it's not about wanking so much as urrgh.
Men sharing porn pics/ clips to 'laugh' at is really common. It's usually content that revolves around a woman having something extreme/ revolting/ degrading done to her.
That however many cups video was a good example of this.
It's really about male bonding over the denigration of women. Haha.
I could be wrong. If they're sending each other things saying this is top wank material then that's even grimmer tbh.
Reading your OP again. Your weird pit in stomach reaction is because they are sharing pics of female degredation for male bonding. Yes it's uncomfortable. It's grim.
In my first office job when the internet was new and people just did whatever even at work. My best friend who was a bloke forwarded some emails from his blokey group at work
I always remember the one that was a pic of a woman (sorry about language) with a really big fish up her cunt with the head sticking out.
I was 23.
I didn't understand why he forwarded it to me. Why they were sharing this.
It wasn't sexual.
Yeah anyway a huge amount of men bond/ enjoy seeing women denigrated.
That is reflected in mainstream porn.
Anyway he told you, he doesn't like it. Your reaction is normal. Say yeah neither of us like it so leave.
If he can say that he's leaving because it's nasty shit then all the better but that's up to him, most wouldn't.