Pope John Paul II: “No circumstance, no purpose, no law whatsoever can ever make licit an act which is intrinsically illicit, since it is contrary to the Law of God which is written in every human heart, knowable by reason itself, and proclaimed by the Church.” (Evangelium Vitae, n. 62).
I think it comes down to whether we discuss on what grounds makes manual stimulation, even mutual stimulation, an illicit sexual act. Honest Question: is it the completion of the illicit act that makes it immoral? Is personal stimulation immoral if only done to climax?
CCC 1756 also rightly highlights that intention alone is not sufficient to make an act moral and that some acts are always gravely illicit. Are unnatural sex acts always gravely illicit regardless of completion or intention?