Skip what you feel. You feel whatever it is when you feel it.
Maybe you even feel annoyed he likes her when YOU do not and you have to deal with the fact that your BF's taste in partners is sometimes meh to you. *shrug*
We don't choose to feel feelings. They just are. What we choose is how to behave in response to those feelings -- react or act with intent.
I suggest you focus more on knowing and articulating your needs.
- What do you need from him that is realistic and reasonable?
- What do you need from yourself that is realistic and reasonable?
And to be honest, on some level I think it probably bugs you that while YOU cannot have more than fun flings on your side of the equation -- he can and is developing something more with this woman you don't even like. So how come you cannot with someone you DO like? For yourself?
Sort that out. Because while he's trying to limit YOU in that arena, you are now wanting to limit HIM in another.
A couple limit you BOTH agree to is an agreement you both agree to.
Limits imposed on others against their will/without their signing up for that? That is going to feel yucky.