What is sex without attachment to sense pleasure actually like?

It might happen that a Noble person gets mistreated against his/her will, not rejoicing even a second, such would be "sex" without attachment, without joy in sensuality. Beside that, all the mystic stories, and excuses of grave addicted, may one put the into the trash and seek after Brahmacariya, 8 precepts, if wishing for paths and fruits after liberation, good householder. Yes, there are many faithful householder who abstain from the low conduct of the villagers... And no, without letting sensuality behind, there is no attainment of any of the four Jhanas. Strong attachment toward this leads downwardly and if one might gain human life, one would not find a pleasing situation, also in regard of inter-relations. Drop it. Not worthy anything at all.

Sex without attachment would be like scratching to feel itchy.

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The Buddha was more a bit more colorful in his teaching and compared the dangers of indiscriminate indulgence in sensual pleasures to embracing a burning bonfire:

AN7.72:1.6: “Mendicants, do you see that bonfire burning, blazing, and glowing?”

Sensual desire is quite powerful. And just like a bonfire, sensual desire also grows when fed and relished. Sensual desire is latent, like a tank of gas waiting for a match. For sexual pleasure in particular, our genes open up and give us access to that tank of gas at a certain age. We can't really be unattached to sensual desire without emptying that tank of gas. Emptying the tank of gas is a bit of work, so putting more gas in the tank to light a fire seems a bit like scratching to get itchy. It would not makes sense.

Fortunately, sensual desire is the fourth fetter. There are three others fetters that precede the fourth fetter and with the relinquishing of each fetter, suffering decreases:

DN33:2.1.20: Five lower fetters:
DN33:2.1.21: identity view, doubt, misapprehension of precepts and observances, sensual desire, and ill will.

It is indeed good to consider others instead of only oneself. When we consider the welfare of others, we can let go of identity view.

I approach this question more from the Theravada tradition as well as my personal understanding. The reason why sex is such a powerful albeit intoxicating experience is because it involves all the senses i.e. sight, sound, smell, taste, touch and mind sense (feelings, perceptions, thoughts and emotions). This essentially creates an immersive sensual experience that is very addictive and, therefore, also very detrimental for the higher attainments as it anchors the mind strongly in the sensual realms. This is perhaps why tantric sex is so alien, in my view, to the Theravada tradition.

As far as the human or animal realm is concerned, I don’t believe it’s possible to experience sex purely through the mind sense i.e. without the pleasant inputs from the five senses of sight, sound, smell, taste and touch. Even in the deva realms (up till kama-loka), there seems to be indication that if sex (or its counterpart) exists, it still involved the five senses as alluded in Sakka’s Palace of Victory.

As already mentioned by the questioner, that clinging to sensual pleasure is one of the five hindrances. It is therefore strange to believe that sex and jhana can co-exist without friction. There is a case mentioned in the sutta of a stream-winner, Isidatta, who was married and non-celibate but upon his death was declared by the Buddha as a once-returner. Still, this seems to indicate it is a rarity rather than a commonality for non-celibacy and the higher attainments (once-returner and above) to happen in a person concurrently.

What is important as mentioned in Migaslasutta, is that one penetrates the Dhamma (glimpse the Deathless) either through intuitive or intellectual (analytical) knowledge. Only then is there certainty upon death that one is reborn in a higher realm.

For monks and nuns, the full time professional practitioners seeking complete liberation from suffering, the advice given for food, craving and conceit is that relying on these, liberation is pursued, but these are to be abandoned upon complete liberation from suffering.

But sex is not relied upon and it's to be abandoned completely in the pursuit of complete liberation from suffering, by monks and nuns.

The Pali suttas have no notion of tantric sex or sex used in a way that leads to enlightenment. In other words, there is no sex without clinging to sensual pleasure, unlike food.

Then Ven. Ananda approached the nun and, on arrival, sat down on a prepared seat. As he was sitting there, he said to the nun: “This body, sister, comes into being through food. And yet it is by relying on food that food is to be abandoned.

“This body comes into being through craving. And yet it is by relying on craving that craving is to be abandoned.

“This body comes into being through conceit. And yet it is by relying on conceit that conceit is to be abandoned.

“This body comes into being through sexual intercourse. Sexual intercourse is to be abandoned. With regard to sexual intercourse, the Buddha declares the cutting off of the bridge.

AN 4.159

What is sex without attachment to sense pleasure actually like?

The sensory experience of sex or masturbation is at its root merely a physical experience. It is a body phenomenon common to humans with bodies. Sexual pleasure and sensations can be used as an object of meditation, not unlike the breath or a candle flame. Most people become absorbed in this merging with the meditation object and may lose awareness that this is all only consciousness and modifications of consciousness. For most humans, there is an arising of grasping when o*g**m approaches. This can be transcended with practice, and the o*g**m may be experienced as "no big deal."

Does it feel different?

I would say yes. Once the dissipative o*g**ms are transcended through yogic practice, regenerative o*g**ms become possible. These can be very different in body location, intensity, duration, and spacious involvement of others, than what most humans experience through common dissipative o*g**ms (those that once you have one, your desire to continue sex is diminished)

Do you only ever have sex to please the other person?

The deeper question is, what serves and liberates love and consciousness? What terms and conditions have you agreed to with your significant other in this area of sex? There are many factors, and in general, I would say "no" to only having sex to please others; however, if you are in a tantric consort relationship, you may, for the sake of growth and experimentation, trust your partner's guidance in this area sometimes more than your own discrimination and personal preferences.

What is being renounced if we continue to engage in sexual relationships?

You are renouncing any hope of sex ever fulfilling your ego, solving dukkha, or changing your basic nature.

Would engagement in sex with oneself (only) be proof that one cannot obtain the jhanas? etc..

No. It is a natural body function. It produces state experiences that may be attractive but do not resolve the existential pain of this realm.

I think you might want to reflect on what sex with attachments is like. Think of the kinds of attachments that commonly result from sexual activity:

• There's greed or hunger, a craving for more, greater, different experiences of pleasure
• There's pride and jealousy, the desire to declare one's conquests or to have what another has already declared
• There's fear — fear of inadequacy, fear of loss, fear of shame, fear of commitment — and fear binds us to fight or flight

The pleasurable aspects of sex is not (in and of itself) problematic. But sexual activity is one of the most karmic activities we can engage in, because it explicitly and implicitly creates changes in our relationship to world around us, with everyone having to adjust to the new conditions. And resistance to change is at the root of all dukkha and tanhā.

Sex in the absence of attachments — well, let's call it 'dharmic' sex, though that term will likely get with disapproval — is actually quite natural. I mean, it's easy to lose track of both your 'self' and your partner's 'self' in the heat of the moment, though it's extremely difficult to keep the egoic self from snapping back into place after the fact (that's the goal of Tantric practices, but one shouldn't adopt those incautiously). At that moment, it isn't about pleasing your partner or yourself, because both of your egoic selves have been set aside. It's merely an open experience of pleasure, in which your pleasure and your partner's are intertwined and indistinguishable. It's not too different from the bliss that arises during jhana, though obviously the latter is more calm and focused, and much easier to maintain. Pleasure is a mental, experience, a result of liberation from the egoic self; the physical pleasure of sex acts as a catalyst for that mental liberation, much as the physical stillness of meditation acts as a catalyst.

If you want to achieve that catalytic effect through sex, you need to establish intimacy and trust with your partner. Intimacy and trust are functions of the egoic mind, yes, but they set the stage for that momentary release of egoic identity, we can't go into sex with fears, worries, and doubts and expect to be transported.