Tell your boyfriend that it's fine if she comes over, on the condition she doesn't fuck you in the ass again. She didn't prepare you, she hurt you, and you won't accept it again. And that is the condition for her EVER coming over again. You have the right to ask for the encounter to be free from pain. Tell him that if she wants to fuck someone in the ass, he has one, too, and that he's welcome to allow it done to himself all he wants.

Be straight and honest with him about what happened and the fact you didn't like it. I would have been livid, myself. I'd never allow her over again just because of that, but then again, I am not interested in other women. If you liked the rest, put your foot down about that.

Pussy= fine
Ass= his only, and even then, gently only

You have that right, hon.

If he insists, then tell him that you want to fuck her ass with a strapon, too, and you need practice. So you need to fuck his ass with a strapon, and he should remember that you didn't get lube, so neither does he. Maybe when his ass clenches up with sympathy pains, he'll be a bit more understanding.