I’m sorry rlg, but I don’t see the logic in his argument. In fact, he doesn’t even present an explanation other than to warn against degradation, impurity and lust.
If I follow his logic, then what would prevent me from using any act I desire as foreplay? Many folks do in fact use clearly disordered acts as a means of preparation for intercourse but that does not neutralize their inherent immorality. I don’t want to get too graphic because these threads keep getting shut down so I will just leave that statement as is and trust you can imagine what I am getting at.
I know you want an answer to your other question and to be honest, I don’t know how to respond. Following the logic I have offered, the answer would be that hand/genital stimulation should fall under the catagory of masturbation. I know that my folks, happily married for 60 years, did not engage in this activity (don’t ask me how I know that

)! I know that their generation (and preceeding generations) considered anything other than the “missionary position” gravely immoral. My question continues to be how, why, and when did this understanding change to what we now have which, at times, appears to be carte blanche in the marital bed.
And if one relies on personal conscience in this matter, due to the lack of clear and distinct teaching, than I would have to say that for me, and many of the men and women of my generation, oral/genital activity has always “felt” immoral and wrong. Before the sexual “revolution” of the 60’s and the radical message of feminism, most men seeking this kind of sexual activity would be forced to seek it among professionals because their wives would not be expected (or agreed!) to do this. So again, my question is when did this change and why?