Why does the Torah add the words "Lo Sikrav"?
Da'as Zekenim and Hadar Zekenim: Seeing as one's wife is available to a man when she is Tehorah, the Torah is warning him not to bring intimacy closer, but to wait until she becomes Tehorah.
Why does the Torah forbid relations with a Nidah?
Ramban, Moshav Zekenim: The Torah permits intimacy only for the sake of childbirth. 1 The blood of Nidus is not capable of forming a baby. 2 And even if it would be formed, the blood would kill it or cause boils or Tzara'as.
Even so, it is permitted in other cases when it is unlikely or impossible to because pregnant, e.g. she is a minor, sterile, after menopause, pregnant or nursing. (PF)
Ramban, Moshav Zekenim: On the contrary, it is poison that kills any creature that drinks it. Moreover, the Nidah herself causes harm to whoever has direct contact with her (See Ramban here and refer to Bereishis 31:35:1:2).
