Why is it that Taharat Hamishpacha is so important and is said to determine the continuation of Am Yisrael?

All Mitzvot are important because they were given by Hashem. Taharat Hamishpacha is no different. However, all Mitzvot of Torah, if conflicted with loss of life or even a risk of life, the life overrides the mitzvah, for instance if a person is going to risk his life if he would not eat non-kosher food, he must eat and live. There are only three Mitzvot a person would rather die or be killed than transgress; there are murder, idolatry and Arayot - prohibited sexual relations. If a woman doesn’t keep the laws of family purity she is Nidah and relations with her husband are classified as Arayot – one of the above three sins she’d rather die than transgress. In that respect – family purity is on a different class. If the penalty for breaking a Mitzvah is so great – the reward of keeping that Mitzvah is great as well.