Natural law ethics essentially states that human beings have functions, each with a purpose. It is morally right to foster these functions, and morally wrong to intentionally frustrate these functions.
Now with this in mind, the natural law ethicist sees the primary purpose of human sexuality as procreation, whereas the secondary purpose (still a good thing) is pleasure and intimacy. As a result, any action that subordinates the primary purpose to the secondary purpose is morally wrong. Therefore, not only are homosexual acts morally wrong, but so are contraception and abortion.
Of course, two objections usually surface around this time.
Objection #1: Wearing eyeglasses and flying planes is unnatural. These actions are morally ambivalent, so something must be wrong with natural law ethics.
Reply to Objection #1: The objection misinterprets the fundamental axiom of natural law ethics. The moral axiom is distinct from the laws of nature, e.g. gravity. Neither of the actions listed as counter-examples results in a frustration of any human function. In fact, eyeglasses actually enhance the purpose of the eyes - namely, eyesight!
Objection #2: If natural law ethics is correct, then the sex of infertile couples is morally wrong. That is absurd.
Reply to Objection #2: Yes, it is absurd, but not for the reason the objector gives. You see, an infertile couple that is male and female are still fertile in kind, even though they are infertile by accident (non-essentially). The infertile couple does not intentionally frustrate the primary function of human sexuality, whereas homosexual acts, contraception and abortion do.