Yet just "no" would be an immoral act itself, since it is discrimination against an act that is morally neutral. Let us leave aside the question of the morality of the act that you hypothesize we might eventually decide to commit, and assume it is immoral. (I phrase it this way since these "unintended consequences" don't follow as a matter of fact but require people to decide to perform them.) Even if we in future might end up deciding to commit an immoral act, that is no justification to continue committing the present immoral act.