I think Bookcat is onto something here. HV states every marriage act must be procreative and unitive. “Marriage act” or “marital act” is broad and includes subsets of acts. The organs must be erect, moist, etc. (w/o getting too graphic). Stimulating the organs so that they may be prepared for the act is a necessary part of the act. Stimulating them outside a completed sex act would be improper.
But Ron Conte will object that the same act can’t be evil in one circumstance and good in another. Actually, they are not the same act. And you must look to context to determine the acts and the term “moral act” may include a subset of acts. And the same act may be evil in one circumstance and good in another.
For instance, if you commit thievery, you might case your neighbor’s house, enter it when you know no one is home, go into the bedroom, take the jewelry, and then leave the house. Thievery was not one act, but a series of acts. It’s a continuum. You would confess “stealing the jewelry” but not confess each and every particular act, b/c they are included in the thievery.
Suppose you go on your neighbor’s property when he told you not to. You’ve committed trespass. Now, suppose that he told you not to come on his property, but you see him face down, and think he’s hurt. You did the same act - going on his property w/o his permission, but you do it to assist or help someone in need. The same act is good, in one circumstance, and bad in another.
Lying is (mostly) bad, but lying to the Nazi’s to preserve the life of the Jews you are hiding is not bad. Lying to Chinese soldiers that you don’t know your ex-wife’s whereabouts (when you do) b/c they are going to forcibly abort her baby is not bad.
Even taking a neighbor’s apple off his property when it won’t threaten or hurt him in any way, so that you can feed your hungry family, is not thievery. That same act may be thievery if you are taking the apple for no good reason.
To recap, murder, thievery, and other sins are sins which capture a whole entire bunch of acts. The “sin” is attributed to the continuum of all these acts.
And an act that is bad, may be good, in different contexts. So, masturbation and contraception are always prohibited. But partially stimulating sexual organs to engage in an act of licit sex is neither masturbatory nor contraceptive.