The burden of proof is with the one who claims that stimulation after the integral/procreative function of the sexual faculty is complete (insofar as that is in the couple’s control) is not autoerotic. In other words, you have to explain why this is not self-abuse… Personally, I do not see the way around the conclusion. There should rather be adequate preparation for the sexual act - but to make up for it afterwards seems rather plainly to be not ordered at all toward procreation, which is an essential element of the right use of venereal pleasure. The female organ exists exclusively to incentivize and aid with the procreative “moment” (by making it more pleasing and by physically preparing the body), wherein other benefits can be found (social, psychological, even physical).
As for the question of the “vas indebitum” - it seems to me that any usage of other parts of the body in preparation for the procreative moment must really and truly be done in view of that moment (really AS preparation), and not done simply for their own sake out of curious pleasure-seeking. This essentially rules out “acts” (protracted amounts of time spent using improper parts of the body) with the male organ - though there could be “moments”.