My boyfriend thinks that he's bisexual

My understanding is that if you are bi you are attracted to both genders. He's not attracted to men. He says he's bisexual because he enjoys pegging/anal stimulating(which I am okay with) I wouldn't care if he was bisexual,I accept him for who he is and that wouldn't change because he's bi. But I'm just confused if my understanding is wrong? I just want to be educated!