Assuming that marital relations with a woman who is already pregnant or post-menopausal do not violate the prohibition of "hashchatat zera" (as there is no chance of resulting pregnancy), what is the reason that there is no violation?

Shalom U'verachah, Wasting zera is relevant in a situation in which zera is ejaculated not during intercourse. However, when there was intercourse even if it cannot lead to pregnancy this is not considered wasting seed. Kol Tuv