Good posting phil,
I have not received any asubha meditation instructions from any teacher, and neither do I practice this regularly.
I find the pictures help in dispelling lust in the 'heat' of the moment, and contemplating the loathsome works well to pour 'cold water' on the moods/feelings spontaneously, I do hope this repeated contemplation loosens a little bit of the sankhara/conditionings, but a long term strategy would probably be better.
For example, avoiding 'exciting foods' like excessive meat, eggs, garlic and ginger, keeping good company and noble friends, avoiding exposure to suggestive material where possible, keeping downcast eyes when pretty women are in full view, learning to use up the extra energy in constructive ways - yoga, light physical exercise etc.
You are correct in that knowing 'why' is more powerful than 'how' on the longer term in maintaining brahmacariya.
One needs to contemplate that to keep brahmacariya is like being Brahma - as divine beings in this realm are sexless, devas and humans otherwise are not immune.
Also, Jhana's will always be out of one's reach. If sensual desire is strong , on retreats and during daily meditation the mind wanders off thinking about the opposite sex, indulging in this fantasy, that fantasy; what a complete waste of time!
One also needs to contemplate the real nature of the body, how the body is like a toilet, we need to keep it nice and clean on the surface, but there is lying as to the stink and filth underneath.
On an funny note - I was reading somewhere that one Thai meditation master mentioned that's what causes males to be reborn as women - excessive lust for the female form!
And your last point is the most valid of all, as ultimately, the biggest sex organ of the human is the mind - this is where the pleasure seeking resides.
According to Ajahn Brahm, developing strong piti sukha and samadhi to Jhana levels gives the mind pleasure and bliss that so much better than sex! The mind at this point doesn't need to 'feed' on anything anymore.
And the bliss of seclusion is something that doesn't depend on anything else, it is something that is blameless, and is something that is close to Nibanna, the unconditioned.
One then naturally drops the desire for sex and adopts brahamacariya for something far superior.
In daily life, catching the defilement with mindfulness/sati sampajanna before it manifests into physical action/ speech is the utmost importance. The question then turns to developing a strong kammatthana or meditation object, whether noting, equanimity in awareness of sensation or mantra in observing the contents of the mind.
There is a good simile I once read , about how a camel poked its head into the owners tent wanting rest from the heat. The owner protested but the camel smooth talked the owner saying "Oh I won't stay long. I want to keep my head out of the sun". But it took a few more steps inside, extending to its neck. Once again the owner protested, but the smooth talking camel again said "Listen I won't stay long, you want me to get a good rest right?". But very soon, the camel had his whole body in the tent and the owner was cramped in the corner, not able to move, or stretch his legs to kick the camel out! The defilements of the mind are like that, so smooth talking, and coming under many disguises, and once it grips the mind, its hard to let it loosen its grip.
So remember, give the camel a good whack on the nose once it peaks its head to say hello! Show him/her who is the boss!
My deep respect for everybody who is making the effort of keeping purity of body and mind.
May we all reach our aspirations this very lifetime.