The Vibhaṅga defines intentionally as “having willed, having made the decision knowingly and consciously.” The Commentary explains these terms as follows: Having willed means having willed, having planned, with the intention of enjoying bringing about an emission. Having made the decision means having summoned up a reckless mind state, “crushing” through the power of an attack. (These are the same terms it uses to explain the same phrase under Pr 3, Pc 61, and Pc 77. The meaning is that one is not simply toying with the idea. One has definitely made up one’s mind to overcome all hesitation by aggressively setting upon an action aimed at causing emission.) Knowingly means knowing that, “I am making an exertion”—which the Sub-commentary explains as knowing that, “I am making an exertion for the sake of an emission.” Consciously means being aware that one’s efforts are bringing about an emission of semen.
The Commentary’s definition of “having willed” is where it deviates from the Vibhaṅga’s discussion of the factor of intention. The Vibhaṅga, throughout its analysis, expresses this factor simply as “aiming at causing an emission,” and it lists ten possible motives for wanting to bring the emission about:
for the sake of health,
for the sake of pleasure,
for the sake of a medicine,
for the sake of a gift (to insects, says the Commentary, although producing semen as a gift to one’s partner in a tantric ritual would also come under this category),
for the sake of merit,
for the sake of a sacrifice,
for the sake of heaven,
for the sake of seed (to produce a child—a bhikkhu who gave semen to be used in artificial insemination would fit in this category),
for the sake of investigating (e.g., to diagnose one’s health), or
for the sake of playfulness or fun.
Each of these motives, the Vibhaṅga says, fulfills the factor of intention here. Thus for the Commentary to limit the question of “deliberate intention” strictly to the enjoyment of the act of bringing about an emission (numbers 2 and 10 in the Vibhaṅga’s list) has no basis in the Canon.
I am skeptical of Thanissaro’s interpretation of what the Commentary means by ‘enjoyment’ as to excluding most of the examples given in the Vibhanga. Obviously the Commentator would not dispute with the list in the Vibhanga.
Perhaps some of the pali scholars can comment.