Why is everyone calling me the selfish one?!?

I've posted a few questions on here about how my bf has asked me several times to have a 3some with me because "I'm the only person he feels comfortable doing it with". He's 24 and said he's always wanted to have one, but I really DON'T want to do it. It would kill me to see him having sex with another girl, and I don't understand how he would be ok with another guy doing whatever he wants with me. So when I say this, people have been calling me a selfish whore, dumb, a slut, all kinds of stuff and I don't understand why! Am I wrong in thinking that he really must not care about me and is just using me for sex/ to turn him on?! I feel like I'm worth more than this..I'm 16 btw.