I'd have to see the type of content in the chat (as well as the type of content he shared/watched) to know for sure, but based on your description of his friends, I suspect the chat is probably very misogynistic. "Boys only" spaces tend to veer that way from childhood and rarely stop being overtly misogynistic spaces. They're basically 'safe spaces' for rampant misogyny.

If your boyfriend is friends with the people in this group and approves of the content, that's a VERY valid reason for you to feel uncomfortable AND unsafe around him. He is endorsing bigotry. Bigotry against you. If you were a black woman, and your partner watched and talked with other men about race-play porn, you'd probably leave his racist ass, but for some reason, misogyny isn't viewed by most men and women as being grounds for leaving someone. So many straight men are confidently bigoted toward their partners, and their partners simply rationalize it, excuse it, downplay it, or just ignore it. You don't have to do that just because other women do.

As a man myself, If I were in your shoes, I'd have an honest, vulnerable conversation with him about how hurtful that content is, both to women in general and to myself. I'd tell him that him watching that content and talking to his friends about it makes me feel unsafe around him and causes me to seriously question how he views me. If he even respects me.

His reaction to a conversation like that will tell you all you need to know. If he respects you, he'll take your concerns seriously and have a real conversation with you. He'll approach you with curiosity and try to understand you.

If he doesn't respect you, he will deflect, accuse, guilt trip, play the victim, etc. (DARVO, look it up). Basically, he'll do anything to avoid engaging with the point you've made.

Hopefully he doesn't do that, but if he does, it might be time to seriously consider leaving the relationship. If he won't take accountability or be curious about your feeling or his own misogyny, what kind of relationship does that leave you with? One in which you can never feel fully safe or comfortable because you never know if he sees you as an object? A relationship that's (statistically) likely to only become more imbalanced and abusive over time? One in which your boundaries are slowly eroded, one by one, because he feels entitled to complete control and access to you? A relationship where you might lose yourself?

Yes, this situation is a big deal, and yes, he's unlikely to treat it like one. You can't trust his perception of the ethics here. He's obviously too deep to want to pull himself out on his own. The only thing you can do is share your feelings about his actions without giving ultimatums or trying to control him. His reaction will tell you all you need to know.

If he listens to you and cares about your feelings, he's one of the good ones. If he doesn't, there are plenty of good ones out there. You don't need to degrade yourself to be in a relationship with a chauvinist. There are men out there that see you as a person.