I thought I'd try writing in response to your questions....I was really tired last evening, and probably did not express myself very well!
I believe that there are a myriad of paths in our spiritual unfoldment, and that each of us will end up on the path that is 'written on the soles of our feet.'
Pir Vilayat Inayat Khan, with whom I studied for 25 years, used to say there are 5 paths to God: The Path of Wisdom (Jnāna), the Path of Devotion (Bhaktī), The Path of Service (Karma), The Path of Renuniciation (Vairāgyā) and The Path of Everyday Life. He would quip that all the Paths lead to God, but the Path of Everyday Life was the most challenging, because it would be so easy to forget that one was on a Path at all!
Your question as I understand it is, "How does sex contribute to our spiritual development?" And I think this question really encapsulates what we both object to in common language around tantra. Because of course, the general understanding of tantra as "the sexy path" ignores the the depth of this embodied tradition, The Path of Everyday Life. It is a limitation which does not feel comfortable.
And yet i have been lost in transcendence (no-time no-space no differentiation between I and Thou) at the altar of maithūna. The transcendence which is exactly the transcendence I have experienced after 40 days of silence, in meditation, fasting from food and drink and sleep. The difference is that the experience was available after only a few hours rather than 40 days.
A short cut on the Path of Everyday Life.
Sexual Intimacy with another person is an opportunity to explore bringing our conscious attention to Our Body and Her pleasure. So it is not sex per se that the Left Hand practices but exquisite attention to The Body within the sexual contact. Like a walking meditation in which we attempt to bring our full awareness with every footfall.
When we have had this experience of sexual union which precedes transcendence, anything other than this in a sexual encounter is simply boring. So this image people carry of Left Handed Tantrikas having random and wanton sex actually is not consistent with my experience.
But, given the Principle of Tantra which states, "Everything is an Experiment," I have played with sex and sexual encounters in a lot of ways most people do not. So this could give an observer the impression that I participate in wanton and random sex.
But, back to your question, "How does sex contribute to our spiritual development?" Shakti (the feminine, Prakriti) is the manifest, and SHE is accessible in all observable (and many unobservable) phenomena. Why are we born into a body? Why do we manifest? If our spiritual development consists of attempting to return to Shiva consciousness, why not stay in Purusha, the archaic, the unmanifest?
This was the push and pull which kept nagging at me, until I began to explore tantra. Why was I born into a body at all, a body which needed to eat, experienced desire, etc., if my purpose was simply to "return to The Oneness," in Sufi terms?
Ibn al-Arabi once said, "THOU didst contrive this I and We in order to play the game of worship with Thyself."
Giving our exquisite attention to Her includes our body, and all forms of manifestation. This is why, I think, as we continue on our path, we become more and more aware of the impact of our actions upon others, and the earth, why we become activists. Recognizing Her capacity for pleasure, and creating opportunities for Her experience of pleasure are ways in which we honor HER, and promote the opportunities for the Inner Marriage, for the Ascending Energy (Shakti) to meet the Descending Consciousness (Shiva).
I hope this begins to answer your question. I should say that what I have written reflects my understanding, and not necessarily those of my teacher. I can only tell you what I have grasped, and Rudy's (Ballentine, my teacher) take on it may be very different, or at least appear to be.
As I said, I have no desire to change you or your practice in any way. Just attempting to throw a bit of light upon your query. And I am open to a more in depth discussion, perhaps when next we meet?
For now much love and gratitude for you, and how you show up in your life,
Arabella