Why can't the descendants of Confucius' seventy-sixth generation marry the descendants of Mencius, Zeng, Kong and Yan?

I am the 74th Confucius. My name is Kong Fan X. We have a family tree and ancestral teachings. Kong cannot marry Zeng. Because in ancient times, Meng Yan was a student of Confucius and studied Confucian ethics, we could not marry our family. I suffered greatly from it. I have a classmate in high school, and his last name used to be because of this. Unfortunately, we have not succeeded now!