My boyfriend is seeing this girl who I really dislike. I’ve told him I don’t like her and I wish he wouldn’t be seeing her. He tells me it’s just sex but he’s also told me I have no right to tell him who he can and cannot date or who he can or cannot sleep with. He even told me he cares more about sex than my opinion which really hurt me. I don’t know what I should do because I feel if I say anything more I will lose him but I’m really hurt by all this.

He told you he cares more about sex than your opinion, and he has made you feel like if you voice any issues you will lose him? That is not a safe or healthy relationship. This is not about whether and how to implement veto powers in a polyamorous relationship - this is about disrespect and cruelty.

You should always feel like you can share your needs with a partner - you do not have the right to demand that they always change to accommodate them, but you do have the right to voice them. A person who makes you fearful of doing that by using implicit or explicit threats to leave and withholding their affection is dangerous and unhealthy. 

You should always feel respected and cared for in a relationship - that does not always mean you get your way, but that your perspective is heard and considered. A person who flat out tells you they do not care about your opinion as compared to their own desires is dangerous and unhealthy.

Stop dating this person. It doesn’t matter who else he is sleeping with or how you feel about them. It does matter how he is treating you: completely inappropriately and unfairly. Run.