Why is it that you are not allowed to be intimate with your wife during niddah, but you are allowed to eat food prepared by a niddah?

Question

I understand the basis from Leviticus for prohibiting intimacy during the niddah period.

The answers to this question say that there is no issue with eating food cooked by a Niddah because "even if we were all pure, one would generally only choose to avoid eating impure foods if one would have needed to remain pure later on for various sacrificial or Temple related rites."

One might argue that:

1. Intimacy with a niddah makes you impure.
2. Eating food cooked by a niddah also makes you impure in exactly the same manner and to the same degree.
3. This impurity is the only reason to abstain from intimacy with a niddah.
4. This impurity is generally not a good reason to abstain from eating food cooked by a niddah until the Temple is rebuilt (and even then only temporarily and in special cases).
5. Hence, we should also generally not abstain from intimacy with a niddah until the Temple is rebuilt (and even then only temporarily and in special cases).

This argument seems flawed, but I don't have the Torah knowledge to explain why it's flawed. Premises 2 and 3 seem the most questionable.

Answer 1

You have correctly identified the flawed premises in your argument.

Premise 2 is technically incorrect. One who is intimate with a niddah has the status of an av hatum'ah (source of impurity) on a biblical level (Hilchot Metam'ei Mishkav uMoshav 3:1). One who eats food touched by a niddah is only considered a sheni letum'ah (derivative of derived impurity) and that only rabbinically (Hilchot She'ar Avot haTum'ah 8:10).

Much more significantly, Premise 3 is not true. In addition to contracting impurity, one who is intimate with a niddah transgresses a prohibition and incurs the penalty of karet (spiritual excision) (Hilchot Issurei Bi'ah 4:1). It is this which prohibits intimacy with a niddah, even in the absence of the Temple.