In Tao, sex is not a taboo; Tao has its own Tantra. The energy of sex has not to be destroyed or repressed, it is not your enemy. It can be transformed, it can become a great help in the search of your ultimateness. So in Zen, the idea of celibacy was dropped. There was no insistence on it, it was your choice, because the question is meditation. If you can meditate and live your life in a natural way, it is acceptable to Tao.
And then another transformation happened: Zen reached from China to Japan, where Shinto, the native religion, was very natural. There it became absolutely affirmative, hence it is not even talked about. There is no need, it is not a question.
You are asking, “Apparently sex was used by some Zen masters – for example, Ikkyu – as a way to transform energy. However, in no translation to date does evidence of this appear.”
That does not mean that sex was a taboo. It was so natural that there was no need to discuss it. You don’t discuss urination. That does not mean you have stopped urinating. You start discussing things only when you start going against nature. If you are natural, there is nothing to discuss.
Life is to live, not to discuss.
Live as deeply and intensely as possible.
Ikkyu is certainly known to have used Tantra as a way of transformation. The sexual energy is nothing but your very life energy, it is only the name. You can call it sex energy, but by your labeling it sex, it does not become different, it is life energy. And it is better to call it life energy, because that is a wider term, more inclusive, more comprehensive.
When you are going deeper into your center, that experience can be explained in many ways. It can be explained the way Hindus have explained it: it is realization of the ultimate, brahmabodh. But brahma is not a person. The word is dangerous; it gives an idea as if we are talking about a person. Brahma is simply the whole energy of the existence.
Jainas will call it self-realization, atmabodh, but their self is not synonymous with the ego. It is synonymous with brahma. You are no longer – in your self-realization you are no longer.
Buddha and Mahavira were contemporaries, and Buddha insisted again and again, that if you are no longer, then why do you call it self-realization? That gives a very distorted description. Call it no-self realization. But Mahavira has his own reasons not to call it no-self realization – people are afraid of no-self realization; if you are going to be nothing, then it is better to remain something. And Mahavira knew that it does not matter whether you call it self-realization or not, you are going to disappear. But keep a positive word which is more attractive.
I can see Mahavira’s compassion in it, but I also can see Buddha’s truthfulness. He says, “If it is really no-self realization, then call it what it is. Don’t deceive people.”
Tantra will call it samadhi.