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:
- Intimacy with a niddah makes you impure.
- Eating food cooked by a niddah also makes you impure in exactly the same manner and to the same degree.
- This impurity is the only reason to abstain from intimacy with a niddah.
- 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).
- 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.