Why do many Jewish and christian parents get angry about interfaith marriages ?

I am a devout believer in Christianity. My friend proposed to this Jewish girl as now her parents won't allow her to marry him because he is a gentile (he's one of those messianic Jews a former Jew who converted to Christianity) I don't understand what the big deal is. A good understanding of Christianity requires and education in both th Torah and Talmud so its not like he is ignorant of their culture. We have to follow the same commandments with the exclusion of the kosher laws (this family isn't even kosher btw). We pray to the same god of the Torah. The only true difference is that christians recognize that we are imperfect that we cannot justify ourselves before god do to out imperfections. God originally dealt with this by the sacrifice system but this could only be done in the temple but since the temple was destroyed 5 years after christs death (as jesus himself said it would be) god had to make a permeant solution in the lord jesus Christ. Christianity was once a sect of Judaism I don't understand why these two people who love eachother pray to the same god follow the same commandments and believe in the same history cannot gain their parents approval