Makes the assumption that the only purpose of sex is reproduction.
Society has moved on, there is now contraception.
Gay people can reproduce, just not naturally.
Natural law fails to keep pace with changes in modern society
The positive effects of a unifying sexual act between a loving couple is a good enough reason for sex without the need to focus on reproduction
Sexual acts often cannot lead to reproduction ie. infertile partners, when a women is pregnant, the menopause
Extramarital sex could be moral under natural law as a way of resolving infertility